Use opportunistic approach for migration testing

Refactored migration tests to use OpportunisticTestCase, removed
unused code, BaseMigrationTestCase class 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,
``openstack_citest`` user credentials will be used only for initial connection
to the database.  TestMigrationUtils class was refactored also, because
BaseMigrationTestCase was removed.

Co-Authored-By: Roman Podoliaka <rpodolyaka@mirantis.com>

Change-Id: I5c9aaa56e5041b919b1e96a19e0395c5e03b727a
Merge "Use opportunistic approach for migration testing"