This commit fixes the following golint warnings:
pkg/api/avatar/avatar.go:229:12: should omit type *http.Client from declaration of var client; it will be inferred from the right-hand side
pkg/login/brute_force_login_protection.go:13:26: should omit type time.Duration from declaration of var loginAttemptsWindow; it will be inferred from the right-hand side
pkg/metrics/graphitebridge/graphite.go:58:26: should omit type []string from declaration of var metricCategoryPrefix; it will be inferred from the right-hand side
pkg/metrics/graphitebridge/graphite.go:69:22: should omit type []string from declaration of var trimMetricPrefix; it will be inferred from the right-hand side
pkg/models/alert.go:37:36: should omit type error from declaration of var ErrCannotChangeStateOnPausedAlert; it will be inferred from the right-hand side
pkg/models/alert.go:38:36: should omit type error from declaration of var ErrRequiresNewState; it will be inferred from the right-hand side
pkg/models/datasource.go:61:28: should omit type map[string]bool from declaration of var knownDatasourcePlugins; it will be inferred from the right-hand side
pkg/plugins/update_checker.go:16:13: should omit type http.Client from declaration of var httpClient; it will be inferred from the right-hand side
pkg/services/alerting/engine.go:103:24: should omit type time.Duration from declaration of var unfinishedWorkTimeout; it will be inferred from the right-hand side
pkg/services/alerting/engine.go:105:19: should omit type time.Duration from declaration of var alertTimeout; it will be inferred from the right-hand side
pkg/services/alerting/engine.go:106:19: should omit type int from declaration of var alertMaxAttempts; it will be inferred from the right-hand side
pkg/services/alerting/notifier.go:143:23: should omit type map[string]*NotifierPlugin from declaration of var notifierFactories; it will be inferred from the right-hand side
pkg/services/alerting/rule.go:136:24: should omit type map[string]ConditionFactory from declaration of var conditionFactories; it will be inferred from the right-hand side
pkg/services/alerting/conditions/evaluator.go:12:15: should omit type []string from declaration of var defaultTypes; it will be inferred from the right-hand side
pkg/services/alerting/conditions/evaluator.go:13:15: should omit type []string from declaration of var rangedTypes; it will be inferred from the right-hand side
pkg/services/alerting/notifiers/opsgenie.go:44:19: should omit type string from declaration of var opsgenieAlertURL; it will be inferred from the right-hand side
pkg/services/alerting/notifiers/pagerduty.go:43:23: should omit type string from declaration of var pagerdutyEventApiUrl; it will be inferred from the right-hand side
pkg/services/alerting/notifiers/telegram.go:21:17: should omit type string from declaration of var telegramApiUrl; it will be inferred from the right-hand side
pkg/services/provisioning/dashboards/config_reader_test.go:11:24: should omit type string from declaration of var simpleDashboardConfig; it will be inferred from the right-hand side
pkg/services/provisioning/dashboards/config_reader_test.go:12:24: should omit type string from declaration of var oldVersion; it will be inferred from the right-hand side
pkg/services/provisioning/dashboards/config_reader_test.go:13:24: should omit type string from declaration of var brokenConfigs; it will be inferred from the right-hand side
pkg/services/provisioning/dashboards/file_reader.go:22:30: should omit type time.Duration from declaration of var checkDiskForChangesInterval; it will be inferred from the right-hand side
pkg/services/provisioning/dashboards/file_reader.go:24:23: should omit type error from declaration of var ErrFolderNameMissing; it will be inferred from the right-hand side
pkg/services/provisioning/datasources/config_reader_test.go:15:34: should omit type string from declaration of var twoDatasourcesConfig; it will be inferred from the right-hand side
pkg/services/provisioning/datasources/config_reader_test.go:16:34: should omit type string from declaration of var twoDatasourcesConfigPurgeOthers; it will be inferred from the right-hand side
pkg/services/provisioning/datasources/config_reader_test.go:17:34: should omit type string from declaration of var doubleDatasourcesConfig; it will be inferred from the right-hand side
pkg/services/provisioning/datasources/config_reader_test.go:18:34: should omit type string from declaration of var allProperties; it will be inferred from the right-hand side
pkg/services/provisioning/datasources/config_reader_test.go:19:34: should omit type string from declaration of var versionZero; it will be inferred from the right-hand side
pkg/services/provisioning/datasources/config_reader_test.go:20:34: should omit type string from declaration of var brokenYaml; it will be inferred from the right-hand side
pkg/services/sqlstore/stats.go:16:25: should omit type time.Duration from declaration of var activeUserTimeLimit; it will be inferred from the right-hand side
pkg/services/sqlstore/migrator/mysql_dialect.go:69:14: should omit type bool from declaration of var hasLen1; it will be inferred from the right-hand side
pkg/services/sqlstore/migrator/mysql_dialect.go:70:14: should omit type bool from declaration of var hasLen2; it will be inferred from the right-hand side
pkg/services/sqlstore/migrator/postgres_dialect.go:95:14: should omit type bool from declaration of var hasLen1; it will be inferred from the right-hand side
pkg/services/sqlstore/migrator/postgres_dialect.go:96:14: should omit type bool from declaration of var hasLen2; it will be inferred from the right-hand side
pkg/setting/setting.go:42:15: should omit type string from declaration of var Env; it will be inferred from the right-hand side
pkg/setting/setting.go:161:18: should omit type bool from declaration of var LdapAllowSignup; it will be inferred from the right-hand side
pkg/setting/setting.go:473:30: should omit type bool from declaration of var skipStaticRootValidation; it will be inferred from the right-hand side
pkg/tsdb/interval.go:14:21: should omit type time.Duration from declaration of var defaultMinInterval; it will be inferred from the right-hand side
pkg/tsdb/interval.go:15:21: should omit type time.Duration from declaration of var year; it will be inferred from the right-hand side
pkg/tsdb/interval.go:16:21: should omit type time.Duration from declaration of var day; it will be inferred from the right-hand side
pkg/tsdb/cloudwatch/credentials.go:26:24: should omit type map[string]cache from declaration of var awsCredentialCache; it will be inferred from the right-hand side
pkg/tsdb/influxdb/query.go:15:27: should omit type *regexp.Regexp from declaration of var regexpOperatorPattern; it will be inferred from the right-hand side
pkg/tsdb/influxdb/query.go:16:27: should omit type *regexp.Regexp from declaration of var regexpMeasurementPattern; it will be inferred from the right-hand side
pkg/tsdb/mssql/mssql_test.go:25:14: should omit type string from declaration of var serverIP; it will be inferred from the right-hand side
