Update Ceph Keyring

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