'Failback host that has been failed back should just return.'

def test_failback_host_that_has_been_failed_back(self):

    self.configuration.replication_device = [TEST_REPLICATION_DEVICE]
    self.driver = FakeDS8KProxy(self.storage_info, self.logger, self.exception, self)
    self.driver.setup(self.ctxt)
    vol_type = volume_types.create(self.ctxt, 'VOL_TYPE', {'replication_enabled': '<is>    True'})
    location = six.text_type({'vol_hex_id': TEST_VOLUME_ID})
    data = json.dumps({'default': {'vol_hex_id': TEST_VOLUME_ID}})
    volume = self._create_volume(volume_type_id=vol_type.id, provider_location=location, replication_driver_data=data)
    (secondary_id, volume_update_list, __) = self.driver.failover_host(self.ctxt, [volume], 'default', [])
    self.assertIsNone(secondary_id)
    self.assertEqual([], volume_update_list)
