Keystone/sql-migrate-extensions

The migrations are performed monolithically, but the separate backends can actually be separately versioned. While this works fine for core, it will not work for extensions. We need to make it simple and clear how to do migrations for extensions. The migrate_version table already supports this: sql> desc migrate_version; +-+--+--+-+-+---+ +-+--+--+-+-+---+ +-+--+--+-+-+---+
 * Field          | Type         | Null | Key | Default | Extra |
 * repository_id  | varchar(250) | NO   | PRI | NULL    |       |
 * repository_path | text        | YES  |     | NULL    |       |
 * version        | int(11)      | YES  |     | NULL    |       |

Each of the backends already support calling the central migrations via the db_sync mechanism.