Port Cisco nexus subplugin UT to ML2 mech driver UT

Port Cisco nexus subplugin UT to ML2 mech driver UT

Fixes bug 1227889

This fix ports the Cisco Nexus sub-plugin unit tests
(neutron/tests/unit/cisco/test_nexus_plugin.py) to the Cisco ML2 mechanism
driver. The new test script will be
neutron/tests/unit/ml2/drivers/test_cisco_nexus.py.

Test cases which will be excluded from this port:
    - Test cases involving provider networks.
    - Test cases involving add/remove router networks
since these features are not currently supported in the Cisco ML2
mechanism driver.

Change-Id: I7aed85031cf402c418089d263c7e949c4631b1eb