Add a base test case for DB schema comparison

Add a base test case that allows to check if the DB schema obtained
by applying of migration scripts is equal to the one produced from
models definitions. It's very important that those two stay in sync
to be able to use models definitions for generation of the initial
DB schema. Though, due to the using of sqlalchemy-migrate (since we
had to write migration scripts manually and alembic can generate
them automatically) we have many divergences in projects right now,
which must be detected and fixed.
Fortunately, we can now rely on alembic + sqlalchemy implementation
of schema comparison instead of writing our own tools.

Blueprint: sync-models-with-migrations
Change-Id: I2d2c35987426dacb1f566569d23a80eee3575a58