Run database migration tests
Run database migration tests

In the new alembic MigrationsWalk unit tests class the migration walk is
wrong:

- It's going in the reverse order
- It's not running the pre upgrade checks
- It's not running the post upgrade check

So the only thing it's doing is run the migration themselves, but since
it's running the latest migration first, we basically just run alls the
migrations at once.

This patch fixes this issue an makes it work as intended, so we can have
the compulsory migration checks and optionally the pre upgrade checks as
well.

Change-Id: Id876cc141c35e3c48f2627813f3f9a6a68254297

openstack/cinder,mahak/cinder,mahak/cinder,openstack/cinder