'Brings an existing backend storage object under Cinder management.
We expect a snapshot name in the existing_ref that matches one in
Purity.'

def manage_existing_snapshot(self, snapshot, existing_ref):

    self._verify_manage_snap_api_requirements()
    self._validate_manage_existing_ref(existing_ref, is_snap=True)
    ref_snap_name = existing_ref['name']
    new_snap_name = self._get_snap_name(snapshot)
    LOG.info('Renaming    existing    snapshot    %(ref_name)s    to    %(new_name)s', {'ref_name': ref_snap_name, 'new_name': new_snap_name})
    self._rename_volume_object(ref_snap_name, new_snap_name, raise_not_exist=True)
    return None
