DB migration tests

DB migration tests

Refactored migration tests to use OpportunisticTestCase, removed
unused code and ``test_migrations.conf`` file.

The main feature of this approach is to create a new database with
random name for each migration test.  This will avoid migration tests of
race conditions and reduce tests intersection. After this change, database
``openstack_citest`` will be used only for initial connection to the database.

``test_migrations.conf`` file not required anymore, because we create test
database for migration test, so we no longer need to keep database credentials.

Implements blueprint: db-migration-tests
Related-bug: #1266595
Change-Id: I4febd485ff53936b636947c86773a23724e24c65