Fix some NVP tests do not do cleanup of nvp ports

Fix some NVP tests do not do cleanup of nvp ports

Some of the NVP test do not execute self.addCleanup(self.fc.reset_all)
from setUp() as NiciraPluginV2TestCase is not the first mixin.
This casuses left over ports to still exist between different testcases.
This patch only fixes this issue for TestNiciraPortsV2 and
NiciraPortSecurityTestCaseas, some of the other tests require the setUp()
for the first mixin to be called for their tests to work.

Note: this is why the tests for the Remove DHCP lease logic are failing.

Fixes bug: 1209355

Change-Id: I932471cdb8925353c5825cd70c8217511d6d7938