Reserve database migrations for stable/icehouse backports

Following the lead of Nova, keystone should "reserve" several migrations numbers in the master branch at the beginning of the Juno cycle for the purpose of backporting database migrations to stable/icehouse if necessary. This should remain true as long as we're using sqlalchemy-migrate instead of alembic.

On two occasions, keystone has sorely needed this functionality (most recently, https://review.openstack.org/#/c/69884/ ), although Nova has never taken advantage of the option (AFAIK).