Port Cisco nexus and network db UT to ML2 mech driver UT

Port Cisco nexus and network db UT to ML2 mech driver UT

Fixes bug 1227964

This fix ports the Cisco Nexus database and network database
unit tests to the Cisco ML2 mechanism driver UT.
The existing test scripts which are ported:
    neutron/tests/unit/cisco/test_nexus_db.py
    neutron/tests/unit/cisco/test_network_db.py
The new test scripts are:
    neutron/tests/unit/ml2/drivers/test_cisco_nexus_db.py
    neutron/tests/unit/ml2/drivers/test_cisco_network_db.py

Note that the QoS database related tests in
test_network_db.py are not ported because QoS is
not currently supported in the Cisco ML2
mechanism driver.

Change-Id: Ib47dab9dafde7c4b8cf0a4930379750f00b9455c