Use opportunistic migrations

Refactored migration tests to use OpportunisticTestCase, removed
unused code, BaseMigrationTestCase class and ``test_migrations.conf``
file.
Added workaround for bug 1359982 to be able to merge this patch.

Closes-Bug: #1359888
Related-Bug: #1359982

Change-Id: Iade2bd2bf50880e167c5a5be8b1236c437f28702