Add support for VM OS image families

[mbrukman]

Since GCE supports image families, we can get rid of logic to specify specific image versions, because using the image family, we can always get the latest version of a particular OS automatically:

Image families simplify the process of managing images in your project by grouping related images together and making it easy to roll forward and roll back between specific image versions. An image family always points to the latest version of an image that is not deprecated. Most public images are grouped into an image families. For example, the debian-8 image family in the debian-cloud project always points to the most recent Debian 8 image.

This will also allow us to drop the caching of OS image lists, and remove a bunch of complexity from configuration.