Tech Debt: Add unit tests to alembic migration code

[Adrian Otto]

Our alembic migration code from https://review.openstack.org/68101 came from Oslo without unit tests. As soon as they are added to Oslo, we can add them to Solum. This bug was created to be sure we do not lose track of this as technical debt.

[Adrian Otto (aotto)]

Is there a bug that tracks this on the Oslo side that we can link to this so we can periodically check if this has been done?

[Pierre Padrixe (stannie) (pierre-padrixe)]

test_migrations.py (the base class to write migration tests) is still not available for Alembic in oslo.
https://github.com/openstack/oslo-incubator/blob/master/openstack/common/db/sqlalchemy/test_migrations.py

So I guess we should use test_migration_base in its alembic modified version which is available in several projects: storyboard, sahara, ironic... (https://github.com/openstack-infra/storyboard/blob/master/storyboard/tests/db/migration/test_migrations_base.py) and once it's finally available in oslo we'll use the oslo version, but anyway the migration that will import the base class have to be written, so let's work on it.

I've some inputs coming from oslo IRC channel and oslo.db code within oslo-incubator is currently frozen as we are in the middle of oslo.db graduation. There is no bp available so I created https://blueprints.launchpad.net/oslo/+spec/db-test-migration-with-alembic