random failures in opencontrail unit tests

[gordon chung]

not a huge deal as it happens pretty infrequently, but i've gotten this error more than once recently.

======================================================================
FAIL: ceilometer.tests.network.statistics.opencontrail.test_client.TestOpencontrailClient.test_vm_statistics_params
tags: worker-3
----------------------------------------------------------------------
Traceback (most recent call last):
  File "ceilometer/tests/network/statistics/opencontrail/test_client.py", line 51, in test_vm_statistics_params
    'virtual_network': 'ccc'})
  File "ceilometer/network/statistics/opencontrail/client.py", line 121, in get_vm_statistics
    resp = self.request(path, fqdn_uuid, data)
  File "ceilometer/network/statistics/opencontrail/client.py", line 54, in request
    self._log_res(resp)
  File "ceilometer/network/statistics/opencontrail/client.py", line 107, in _log_res
    LOG.debug(''.join(dump))
TypeError: sequence item 3: expected string, MagicMock found
Traceback (most recent call last):
_StringException: Empty attachments:
  stderr
  stdout