'Update the consistency group.
Expected format of the input parameter \'body\':
.. code-block:: json
"consistencygroup":
"name": "my_cg",
"description": "My consistency group",
"add_volumes": "volume-uuid-1,volume-uuid-2,...",
"remove_volumes": "volume-uuid-8,volume-uuid-9,..."'

def update(self, req, id, body):

    LOG.debug('Update    called    for    consistency    group    %s.', id)
    if (not body):
        msg = _('Missing    request    body.')
        raise exc.HTTPBadRequest(explanation=msg)
    self.assert_valid_body(body, 'consistencygroup')
    context = req.environ['cinder.context']
    consistencygroup = body.get('consistencygroup', None)
    self.validate_name_and_description(consistencygroup)
    name = consistencygroup.get('name', None)
    description = consistencygroup.get('description', None)
    add_volumes = consistencygroup.get('add_volumes', None)
    remove_volumes = consistencygroup.get('remove_volumes', None)
    allow_empty = self._check_update_parameters_v3(req, name, description, add_volumes, remove_volumes)
    self._update(context, id, name, description, add_volumes, remove_volumes, allow_empty)
    return webob.Response(status_int=http_client.ACCEPTED)
