CloudWatch: Fix alerting for queries with Id (using GetMetricData) (#17899)

This commit addresses half of #13749 by making sure GetMetricData 
works for alerting. Math Expressions (compound metrics) will still not 
work for alerting, this would require a bigger refactoring of Grafana's 
alerting service. However, with this commit at least alerting for basic 
metrics with non empty query Id will work.

Fixes half of #13749

pkg/tsdb/cloudwatch/get_metric_data_test.go

@@ -31,7 +31,6 @@ func TestCloudWatchGetMetricData(t *testing.T) {
 					Period:     300,
 					Id:         "id1",
 					Expression: "",
-					ReturnData: true,
 				},
 				"id2": {
 					RefId:      "B",
@@ -39,7 +38,6 @@ func TestCloudWatchGetMetricData(t *testing.T) {
 					Statistics: []*string{aws.String("Average")},
 					Id:         "id2",
 					Expression: "id1 * 2",
-					ReturnData: true,
 				},
 			}
 
@@ -58,11 +56,9 @@ func TestCloudWatchGetMetricData(t *testing.T) {
 					So(*v.MetricStat.Period, ShouldEqual, 300)
 					So(*v.MetricStat.Stat, ShouldEqual, "Average")
 					So(*v.Id, ShouldEqual, "id1")
-					So(*v.ReturnData, ShouldEqual, true)
 				} else {
 					So(*v.Id, ShouldEqual, "id2")
 					So(*v.Expression, ShouldEqual, "id1 * 2")
-					So(*v.ReturnData, ShouldEqual, true)
 				}
 			}
 		})