use default min interval of 1m for sql datasources

public/app/plugins/datasource/mssql/datasource.ts

@@ -12,7 +12,7 @@ export class MssqlDatasource {
     this.name = instanceSettings.name;
     this.id = instanceSettings.id;
     this.responseParser = new ResponseParser(this.$q);
-    this.interval = (instanceSettings.jsonData || {}).timeInterval;
+    this.interval = (instanceSettings.jsonData || {}).timeInterval || '1m';
   }
 
   interpolateVariable(value, variable) {

public/app/plugins/datasource/mysql/datasource.ts

@@ -15,7 +15,7 @@ export class MysqlDatasource {
     this.id = instanceSettings.id;
     this.responseParser = new ResponseParser(this.$q);
     this.queryModel = new MysqlQuery({});
-    this.interval = (instanceSettings.jsonData || {}).timeInterval;
+    this.interval = (instanceSettings.jsonData || {}).timeInterval || '1m';
   }
 
   interpolateVariable = (value, variable) => {

public/app/plugins/datasource/postgres/datasource.ts

@@ -17,7 +17,7 @@ export class PostgresDatasource {
     this.jsonData = instanceSettings.jsonData;
     this.responseParser = new ResponseParser(this.$q);
     this.queryModel = new PostgresQuery({});
-    this.interval = (instanceSettings.jsonData || {}).timeInterval;
+    this.interval = (instanceSettings.jsonData || {}).timeInterval || '1m';
   }
 
   interpolateVariable = (value, variable) => {