tempest test_create_router_set_gateway_with_fixed_ip often fails with DVR

test_create_router_set_gateway_with_fixed_ip often fails in neutron-tempest-dvr-ha-multinode-full:

traceback-1: {{{
Traceback (most recent call last):
  File "/opt/stack/tempest/tempest/lib/common/utils/test_utils.py", line 87, in call_and_ignore_notfound_exc
    return func(*args, **kwargs)
  File "/opt/stack/tempest/tempest/lib/services/network/networks_client.py", line 52, in delete_network
    return self.delete_resource(uri)
  File "/opt/stack/tempest/tempest/lib/services/network/base.py", line 41, in delete_resource
    resp, body = self.delete(req_uri)
  File "/opt/stack/tempest/tempest/lib/common/rest_client.py", line 329, in delete
    return self.request('DELETE', url, extra_headers, headers, body)
  File "/opt/stack/tempest/tempest/lib/common/rest_client.py", line 702, in request
    self._error_checker(resp, resp_body)
  File "/opt/stack/tempest/tempest/lib/common/rest_client.py", line 823, in _error_checker
    raise exceptions.Conflict(resp_body, resp=resp)
tempest.lib.exceptions.Conflict: Conflict with state of target resource
Details: {'type': 'NetworkInUse', 'message': 'Unable to complete operation on network 40a63562-61e7-41b0-82c8-e076b8463584. There are one or more ports still in use on the network.', 'detail': ''}
}}}

Traceback (most recent call last):
  File "/opt/stack/tempest/tempest/lib/common/utils/test_utils.py", line 87, in call_and_ignore_notfound_exc
    return func(*args, **kwargs)
  File "/opt/stack/tempest/tempest/lib/services/network/subnets_client.py", line 52, in delete_subnet
    return self.delete_resource(uri)
  File "/opt/stack/tempest/tempest/lib/services/network/base.py", line 41, in delete_resource
    resp, body = self.delete(req_uri)
  File "/opt/stack/tempest/tempest/lib/common/rest_client.py", line 329, in delete
    return self.request('DELETE', url, extra_headers, headers, body)
  File "/opt/stack/tempest/tempest/lib/common/rest_client.py", line 702, in request
    self._error_checker(resp, resp_body)
  File "/opt/stack/tempest/tempest/lib/common/rest_client.py", line 823, in _error_checker
    raise exceptions.Conflict(resp_body, resp=resp)
tempest.lib.exceptions.Conflict: Conflict with state of target resource
Details: {'type': 'SubnetInUse', 'message': 'Unable to complete operation on subnet a1110e0b-d7c8-4830-b1df-e526b632aab9: One or more ports have an IP allocation from this subnet.', 'detail': ''}