Cloudwatch: Fix Unexpected error (#74420)

Fix unexpected error when creating a new cloudwatch datasource.

Involves a fair amount of refactoring, so if this causes unexpected issues related to region fetching we can turn this off with the cloudwatchNewRegionsHandler feature toggle, although we do not predict it will so we are enabling it to default to true and hope to remove it shortly.

pkg/tsdb/cloudwatch/test_utils.go

@@ -135,14 +135,15 @@ func (c *mockEC2Client) DescribeInstancesPages(in *ec2.DescribeInstancesInput, f
 	return args.Error(0)
 }
 
-type fakeEC2Client struct {
+// Please use mockEC2Client above, we are slowly migrating towards using testify's mocks only
+type oldEC2Client struct {
 	ec2iface.EC2API
 
 	regions      []string
 	reservations []*ec2.Reservation
 }
 
-func (c fakeEC2Client) DescribeRegions(*ec2.DescribeRegionsInput) (*ec2.DescribeRegionsOutput, error) {
+func (c oldEC2Client) DescribeRegions(*ec2.DescribeRegionsInput) (*ec2.DescribeRegionsOutput, error) {
 	regions := []*ec2.Region{}
 	for _, region := range c.regions {
 		regions = append(regions, &ec2.Region{
@@ -154,7 +155,7 @@ func (c fakeEC2Client) DescribeRegions(*ec2.DescribeRegionsInput) (*ec2.Describe
 	}, nil
 }
 
-func (c fakeEC2Client) DescribeInstancesPages(in *ec2.DescribeInstancesInput,
+func (c oldEC2Client) DescribeInstancesPages(in *ec2.DescribeInstancesInput,
 	fn func(*ec2.DescribeInstancesOutput, bool) bool) error {
 	reservations := []*ec2.Reservation{}
 	for _, r := range c.reservations {