Implement cinder client plugin

This moves the client creation code out of Clients._cinder() into
its own client plugin.

Cinder is a core project, and python-cinderclient is a dependency,
so the import is now mandatory (including the volume_backups import).

Change-Id: I6b937422cd8e20fa620a7e875ae8b4c61e8f9cf8
Merge "Implement cinder client plugin"