Make test service_plugins override simple
Make test service_plugins override simple

Just make things simple and give a chance to accept that
ordered 'service_plugins' list or tuple for the base test
class NeutronDbPluginV2TestCase.

Change-Id: I7c352ede811703e3a96848378b92cbbf5e109228
Related-Bug: #1809238

mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron,mahak/neutron