'Manage an existing LeftHand snapshot.
existing_ref is a dictionary of the form:
{\'source-name\': <name of the snapshot>}'

@cinder_utils.trace
def manage_existing_snapshot(self, snapshot, existing_ref):

    self._check_api_version()
    volume = snapshot['volume']
    if (volume.get('replication_status') == 'failed-over'):
        err = _('Managing    of    snapshots    to    failed-over    volumes    is    not    allowed.')
        raise exception.InvalidInput(reason=err)
    target_snap_name = self._get_existing_volume_ref_name(existing_ref)
    client = self._login()
    try:
        updates = self._manage_snapshot(client, volume, snapshot, target_snap_name, existing_ref)
    finally:
        self._logout(client)
    return updates
