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
Convert all incoming protocol numbers to string

PostgreSQL is more sensitive for types than MySQL when it selects
something from columns in database. So it fails when it tries to
select from string field comparing with integer value.

Added unit test to verify conversion of protocol numbers to
strings.

Closes-bug:#1381379

Change-Id: I0a29595403a07c66888871088d5549705a097f68
Merge "Use correct base class for unit tests for ML2 drivers"
Merge "Convert all incoming protocol numbers to string"