Direct move of the revoke model from keystone server

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
Merge "Direct move of the revoke model from keystone server"