Add support for volume backups

Implements support for the volume backup api added to cinder in
https://review.openstack.org/19468. This is a resubmit of the
expired change https://review.openstack.org/25299 with the
following changes:

* Added unit tests for backups (v1 and v2)
* Changed references in backup code to display_name and
  display_description to name and description respectively
* Removed links from backup-show output
* Added object_count to _translate_backup_keys
* Removed unneccesary items from _translate_backup_keys
* Fixed backups docstrings including removing references to swift.

Adds backup-create, backup-delete, backup-list, backup-restore
and backup-show to both the v1 and v2 clients, since the volume
backup extension is available via both APIs.

Change-Id: I197384f1c2fd2af641d207a5f4dba0dfbc5c681a