Nicira test_looping_calls periodic failures in stable/havana

[Adam Gandelman]

Hit this 2/3 times while trying to get a trivial version change thru the review checks @ https://review.openstack.org/#/c/89441/.

ft1.5395: neutron.tests.unit.nicira.test_nvp_sync.SyncLoopingCallTestCase.test_looping_calls_StringException: Empty attachments:
  pythonlogging:''
  stderr
  stdout

Traceback (most recent call last):
  File "neutron/tests/unit/nicira/test_nvp_sync.py", line 254, in test_looping_calls
    5, synchronizer._synchronize_state.call_count)
  File "/home/jenkins/workspace/gate-neutron-python26/.tox/py26/lib/python2.6/site-packages/testtools/testcase.py", line 321, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/home/jenkins/workspace/gate-neutron-python26/.tox/py26/lib/python2.6/site-packages/testtools/testcase.py", line 406, in assertThat
    raise mismatch_error
MismatchError: 5 != 2

[Eugene Nikanorov (enikanorov)]

As we approach Juno we might not want to make UT fixes to branch as old as havana.
Marking as Won't fix