Use opportunistic approach for migration testing

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

This change allows tests use database ``openstack_citest`` only
for connection to the database backend. The main feature of this
approach is - for each migration test will be created new database
with random name. This will avoid migration tests of race conditions
and reduce tests intersection.

``test_migrations.conf`` file was removed, because we create test
database for migration test, so we no longer need test database credentials.

Closes-Bug: #1327397
Closes-Bug: #1328997

Change-Id: I95ad140ba5f483cd3dc36e2b78f140826d57624f
Merge "Use opportunistic approach for migration testing"