refactor LoadBalancerPluginDbTestCase setUp()

fixes bug: 1129417

This fixes the bug by changing LoadBalancerPluginDbTestCase to inherit
from test_db_plugin.QuantumDbPluginV2TestCase.  This allows for the
plugin to access context managers for core resources.  Additionally,
this change adds subresource and service_plugin support to
QuantumDbPluginV2TestCase.

Change-Id: I5e838a3df6d4c7192d4a738d3860f3a162cb5a8c
Merge "refactor LoadBalancerPluginDbTestCase setUp()"