Move base migration test classes to common code

Moved base migration test classes WalkVersionsMixin(),
BaseMigrationTestCase() to openctack common module
openstack.common.db.sqlalchemy.test_migrations.
Tests modified.

Part of blueprint common-unit-tests

Change-Id: I415e11864a1305cf074f2685eb610ae8ce96e8c5
Merge "Move base migration test classes to common code"