Add tests for kops alpha channel
Add tests for kops alpha channel

The alpha channel has the upcoming cloud images, so we can catch issues
before images are committed to the stable channel.

This will also let us gather data for issues which may be image
dependent e.g.
https://github.com/kubernetes/kubernetes/issues/58578

GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid