Use monkey_patch() in TestMigrationUtils setUp()

We use monkey patched sqlalchemy-migrate in
openstack.common.db.sqlalchemy.utils module. It allows us drop
unique constrains is SQLite.
If we run all tests, we also use `patched` migrate in
TestMigrationUtils(), but it patches in
`tests/unit/db/sqlalchemy/test_migrate.py` file. So tests from
TestMigrationUtils() class fails if we run them separately.

Added sqlalchemy-migrate monkey patch in setUp() method
TestMigrationUtils test class due to use `patched` migrate
in tests.

Fixes bug 1217306

Change-Id: I037ad36c1db3f88501f2b0f6ae9f2c946de1afa6
Merge "Use monkey_patch() in TestMigrationUtils setUp()"