Explicitly specify default region in CloudWatch datasource (#9440)

The datasource uses the default region in the query if the region
is "" in the settings. However setting the region to an empty string
is almost impossible and rendered incorrectly.

This commit introduces a special value "default" for region which
is shown in the drop down and is translated to the default region
of the data source when performing queries.

pkg/tsdb/cloudwatch/credentials.go

@@ -141,6 +141,11 @@ func ec2RoleProvider(sess *session.Session) credentials.Provider {
 }
 
 func (e *CloudWatchExecutor) getDsInfo(region string) *DatasourceInfo {
+	defaultRegion := e.DataSource.JsonData.Get("defaultRegion").MustString()
+	if region == "default" {
+		region = defaultRegion
+	}
+
 	authType := e.DataSource.JsonData.Get("authType").MustString()
 	assumeRoleArn := e.DataSource.JsonData.Get("assumeRoleArn").MustString()
 	accessKey := ""