Revive Alembic-based database migration logic.

Alembic migrations were introduced to Barbican a while ago but have not been
used recently. This CR revisits the migration logic and ensures that time-zero
database tables creation is distinguished from Alembic-base schema updates.
The associated database migration script is also revived, allowing for
schema migrations outside of the Barbican boot process.

Change-Id: I3e7fd7ac3f629da18329c22ad11cb9ccc1b7f9f9
Implements: blueprint db-revive-migrations
Merge "Revive Alembic-based database migration logic."