`revocation_event` model class missed index at revoked_at column, whereas a
migration keystone/contrib/revoke/migrate_repo/versions/001_revoke_table.py:35
provides the index. The migration looks correct, but the model doesn't
appropriate to the migration.
Closes-Bug: #1397899
Change-Id: I5205f9a0e951d43badd2b11fd5b20a5d59c07ee8