Fix DB migrations in unit tests

The next (very much required) step would be to include
https://review.openstack.org/#/c/74081/
It seems to be about ready to be used by StoryBoard.

Before this patch:
* unit tests used Alembic migrations to populate the database
* unit tests were ran on SQLite which caused a lot of trouble, because of lack
  of support for ALTER operations

After this patch:
* DB will be populated from SQLAlchemy metadata for each test
* unit tests will still use SQLite
* migrations will be tested against MySQL and Postgres
* Developers are required to have local MySQL and Postgres, otherwise tests
  will timeout after 60 seconds (see .testr.conf). It seems like oslo update
  changed behavior, before that update opportunistic tests failed immediately

As a proof of concept this patch includes:
https://review.openstack.org/#/c/79208/

Change-Id: Ia90a30aa8b09fa19bf2819d82d91d3483ed68886