Use correct base class for unit tests for ML2 drivers

Use correct base class for unit tests for ML2 drivers

In unit tests for several ML2 mechanism drivers (listed below)
the NeutronDbPluginV2TestCase class is used instead of Ml2PluginV2TestCase
which is desired to be used.

Unit tests for ML2 mechanism drivers in neutron/tests/unit/ml2:
   drivers/cisco/nexus/test_cisco_mech.py
   drivers/brocade/test_brocade_mechanism_driver.py
   drivers/freescale/test_mechanism_fslsdn.py
   test_mechanism_ncs.py
   test_mechanism_odl.py
   test_l2population.py

Partial-Bug: #1373153

Change-Id: I9524cafb39975b1384954a2eefcbf754231d2198