can't drop sub-schema from version control

[Jamie Lennox]

In keystone we have a number of extensions that all have there own migrate_repos in the extension folder, which get added with their own key to the migrate_versions table when you db_version_control them [1]. This allows multiple schemas to be registered to the same database.

The inverse of this action drops the entire migrate_versions table [2] meaning that once registered you cannot remove a single schema from version control.

We need this behaviour as to migrate an extension into the core of our application we want to bring over the old migrations into the standard migrations, which are only run if the extension migration version is less than expected, and then remove only that extension from version control.

[1] https://github.com/stackforge/sqlalchemy-migrate/blob/a03b141a954c7e644f0033defdb1b5b434a7c49a/migrate/versioning/schema.py#L165
[2] https://github.com/stackforge/sqlalchemy-migrate/blob/a03b141a954c7e644f0033defdb1b5b434a7c49a/migrate/versioning/schema.py#L59