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
Fix error count for stack-list while show deleted

1.The count for stack-list is incorrect while show deleted.
  The count should include the deleted ones.
2.We should check the value of 'with_count', if the value
  is invalid, we should raise ValueError.

Change-Id: Ie1eef7a7ec63870ac28929bcadcd917bbc8d3203
Closes-bug: #1331383
Fix OS::Trove::Instance restore_point property

python-troveclient expects restorePoint to be a dictionary with a
"backupRef" key.

Change-Id: Ib4d83e82dcd5c012fdc12255a93682a6c2d9f47c
Closes-Bug: #1338626
Merge "Implement cinder client plugin"
Merge "Fix OS::Trove::Instance restore_point property"
Merge "Fix error count for stack-list while show deleted"