Migrate default extensions

When running `keystone-manage db_sync` with no parameters, none of
the extensions are migrated.

This patch will now, by default, migrate the following extensions:
  revoke

DocImpact
Co-Authored-By: Steve Martinelli <stevemar@ca.ibm.com>
Closes-Bug: 1324260
Change-Id: I4711e8670bf9619b9bccc09bd6a843728230b25f
Merge "Migrate default extensions"