Refactor test_migrations module

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.

Partial-Bug: #1368274

Change-Id: Ib1d6dfae62cc60f814c01d07adc53f68e7c234f6
Merge "Refactor test_migrations module"