test_hotplug_nic failure ratio is high

it seems that test_hotplug_nic is failing rather frequently.

eg. http://logs.openstack.org/43/507343/4/gate/gate-grenade-dsvm-networking-midonet-ml2-ubuntu-xenial/7b52ab9/logs/testr_results.html.gz

traceback-1: {{{
Traceback (most recent call last):
  File "tempest/lib/common/utils/test_utils.py", line 84, in call_and_ignore_notfound_exc
    return func(*args, **kwargs)
  File "tempest/lib/services/network/subnets_client.py", line 52, in delete_subnet
    return self.delete_resource(uri)
  File "tempest/lib/services/network/base.py", line 41, in delete_resource
    resp, body = self.delete(req_uri)
  File "tempest/lib/common/rest_client.py", line 301, in delete
    return self.request('DELETE', url, extra_headers, headers, body)
  File "tempest/lib/common/rest_client.py", line 659, in request
    self._error_checker(resp, resp_body)
  File "tempest/lib/common/rest_client.py", line 780, in _error_checker
    raise exceptions.Conflict(resp_body, resp=resp)
tempest.lib.exceptions.Conflict: An object with that identifier already exists
Details: {u'detail': u'', u'message': u'Unable to complete operation on subnet 8800fca6-df5d-4cb3-a59d-14bd2efd42f7: One or more ports have an IP allocation from this subnet.', u'type': u'SubnetInUse'}
}}}

traceback-2: {{{
Traceback (most recent call last):
  File "tempest/lib/common/utils/test_utils.py", line 84, in call_and_ignore_notfound_exc
    return func(*args, **kwargs)
  File "tempest/lib/services/network/networks_client.py", line 52, in delete_network
    return self.delete_resource(uri)
  File "tempest/lib/services/network/base.py", line 41, in delete_resource
    resp, body = self.delete(req_uri)
  File "tempest/lib/common/rest_client.py", line 301, in delete
    return self.request('DELETE', url, extra_headers, headers, body)
  File "tempest/lib/common/rest_client.py", line 659, in request
    self._error_checker(resp, resp_body)
  File "tempest/lib/common/rest_client.py", line 780, in _error_checker
    raise exceptions.Conflict(resp_body, resp=resp)
tempest.lib.exceptions.Conflict: An object with that identifier already exists
Details: {u'detail': u'', u'message': u'Unable to complete operation on network 454375c8-5734-4e13-a07c-e1d8d9277f6c. There are one or more ports still in use on the network.', u'type': u'NetworkInUse'}
}}}

Traceback (most recent call last):
  File "tempest/lib/common/rest_client.py", line 871, in wait_for_resource_deletion
    raise exceptions.TimeoutException(message)
tempest.lib.exceptions.TimeoutException: Request timed out
Details: (TestNetworkBasicOps:_run_cleanups) Failed to delete resource ff2ecc09-7e40-4329-b59d-a59705e92521 within the required time (300 s).