This commit copies the revoke model from Keystone to
keystoneclient. The only difference in the model.py
file between keystoneclient and keystone is the
import due to a change of the oslo-incubator path
(project name is keystoneclient not keystone).
Blueprint: revocation-event-api
Change-Id: I6215d6679f9bb0dff6941c2a65ceeefd1ff9d88a