'Updates a policy.
For a full list of available parameters, please refer to the official
API reference:
http://developer.openstack.org/api-ref/identity/v3/index.html#update-policy'

def update_policy(self, policy_id, **kwargs):

    post_body = json.dumps({'policy': kwargs})
    url = ('policies/%s' % policy_id)
    (resp, body) = self.patch(url, post_body)
    self.expected_success(200, resp.status)
    body = json.loads(body)
    return rest_client.ResponseBody(resp, body)
