fix cloudwatch ec2_instance_attribute (#9718)

* fix cloudwatch ec2_instance_attribute

* add test

* minor fix

(cherry picked from commit 52e1c4113b)

pkg/tsdb/cloudwatch/cloudwatch.go

@@ -17,6 +17,7 @@ import (
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/request"
 	"github.com/aws/aws-sdk-go/service/cloudwatch"
+	"github.com/aws/aws-sdk-go/service/ec2/ec2iface"
 	"github.com/grafana/grafana/pkg/components/null"
 	"github.com/grafana/grafana/pkg/components/simplejson"
 	"github.com/grafana/grafana/pkg/metrics"
@@ -24,6 +25,7 @@ import (
 
 type CloudWatchExecutor struct {
 	*models.DataSource
+	ec2Svc ec2iface.EC2API
 }
 
 type DatasourceInfo struct {