[google compute] improved Images coverage

Updates to improve the `GCENodeImage` coverage of the GCE API.  Specifically, this update has:
 * Added object attributes to match the upstream Image resource (e.g. `diskSizeGb`, `sourceType`, etc)
 * Moving toward deprecating older attributes such as `preferredKernel`. This has long been removed from the GCE API, but there is a chance very old customer images still retain this value.
 * Add support for setting timestamps for deprecating images for when they would will be automatically marked as `DELETED`, `OBSOLETE`, and `DEPRECATED`.
 * Also adding a nice-to-have feature for allowing users to specify a search-list of projects versus just the user's project and hard-coded list of supported image projects.

No breaking changes introduced and tests have been enhanced for these changes.
If it's not clear, I'm ensuring that the string can be properly converted but I don't actually want to use the generated timestamp.  The flake8 tests don't like unused variables so I was suppressing it's check on these lines.