Use OVO in ml2/test_db
Use OVO in ml2/test_db

Migrate to using Port OVO in the ML2 database testing, specifically in
Ml2DvrDBTestCase.

Partially-Implements: blueprint adopt-oslo-versioned-objects-for-db
Change-Id: Ic7d31f4853776a641283ecb94d4357bb71cd2419

huntxu/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,huntxu/neutron,noironetworks/neutron