return default region list from backend

public/app/plugins/datasource/cloudwatch/config_ctrl.ts

@@ -41,32 +41,7 @@ export class CloudWatchConfigCtrl {
     { name: 'Yearly', value: 'Yearly', example: '[logstash-]YYYY' },
   ];
 
-  regions = [
-    'ap-northeast-1',
-    'ap-northeast-2',
-    'ap-northeast-3',
-    'ap-south-1',
-    'ap-southeast-1',
-    'ap-southeast-2',
-    'ca-central-1',
-    'cn-north-1',
-    'cn-northwest-1',
-    'eu-central-1',
-    'eu-north-1',
-    'eu-west-1',
-    'eu-west-2',
-    'eu-west-3',
-    'me-south-1',
-    'sa-east-1',
-    'us-east-1',
-    'us-east-2',
-    'us-gov-east-1',
-    'us-gov-west-1',
-    'us-iso-east-1',
-    'us-isob-east-1',
-    'us-west-1',
-    'us-west-2',
-  ];
+  regions = [];
 
   getRegions() {
     this.datasourceSrv
@@ -74,10 +49,13 @@ export class CloudWatchConfigCtrl {
       .then(ds => {
         return ds.getRegions();
       })
-      .then(regions => {
-        this.regions = _.map(regions, 'value');
-      }, err => {
-        console.error('failed to call ec2:DescribeRegions, please check iam setting');
-      });
+      .then(
+        regions => {
+          this.regions = _.map(regions, 'value');
+        },
+        err => {
+          console.error('failed to get latest regions');
+        }
+      );
   }
 }