RBD cleanup

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
keyring: use exec to manage cephkeyring group

- Add 'nova' user to 'cephkeyring' group with ensure_resource
- Add 'cinder' user to 'cephkeyring' group with ensure_resource
- Update fixtures & Puppetfile to lastest commit in puppet-cinder
  stable/havana
- Add a unit test with multiple RBD backends
Merge pull request #419 from enovance/update-cinder

keyring: use exec to manage cephkeyring group