Security: Store datasource passwords encrypted in secureJsonData (#16175)

* Store passwords in secureJsonData

* Revert unnecessary refactors

* Fix for nil jsonSecureData value

* Remove copied encryption code from migration

* Fix wrong field reference

* Remove migration and provisioning changes

* Use password getters in datasource proxy

* Refactor password handling in datasource configs

* Add provisioning warnings

* Update documentation

* Remove migration command, moved to separate PR

* Remove unused code

* Set the upgrade version

* Remove unused code

* Remove double reference

devenv/datasources.yaml

@@ -22,10 +22,11 @@ datasources:
     access: proxy
     database: site
     user: grafana
-    password: grafana
     url: http://localhost:8086
     jsonData:
       timeInterval: "15s"
+    secureJsonData:
+      password: grafana
 
   - name: gdev-opentsdb
     type: opentsdb
@@ -110,14 +111,16 @@ datasources:
     url: localhost:3306
     database: grafana
     user: grafana
-    password: password
+    secureJsonData:
+      password: password
 
   - name: gdev-mysql-ds-tests
     type: mysql
     url: localhost:3306
     database: grafana_ds_tests
     user: grafana
-    password: password
+    secureJsonData:
+      password: password
 
   - name: gdev-mssql
     type: mssql