Refactor ResourceGroup's FnGetAtt()

Change-Id: I424e4ce67bbc04739e028dfbe197595e4c6254f8
Move glance_utils functions to glance client plugin

The glance client plugin is a more obvious home for this sort
of function, and this change makes mocking of these functions
much easier since a mocked client is no longer needed.

To access the methods defined in the client plugin the method
Resource.client_plugin has been added. When all clients are
contributed by plugins then method Resource.client will call
Resource.client_plugin.

glance_utils can be deleted since it was only added at the
beginning of the Juno cycle.

The image constraint has also been moved to the glance plugin.

Change-Id: Id31ccd605411ca034addc67d8aadf7cea0904bfd
Merge "Refactor ResourceGroup's FnGetAtt()"
Merge "Move glance_utils functions to glance client plugin"