Move DB TestModelsMigrations from unit to functional

Move DB TestModelsMigrations from unit to functional

The tests to check that DB migrations and models are in sync depends
on the mysql and postgresql backends being available with a specific
DB user and database created. This violates the principles for unit
tests and therefore these tests should be moved to functional tests.

For these tests to work in the functional job in the gate, the
backends must be installed and the DB user and database created.
We do this via the functional gate hook.

Closes-bug: #1372981

Change-Id: I7b89feef3f19ca07dbfb05acfaa30529a5bf683d