Description of problem:

Fail to run all the tests under 'tempest.api.network.test_dhcp_ipv6.NetworksTestDHCPv6'.

Version-Release number of selected component (if applicable): 
openstack-neutron-9.0.0-0.20160929051647.71f2d2b.el7ost.noarch


How reproducible: 100%


Steps to Reproduce:
1. Deploy RHOSP 10 HA
2. Run Tempest API DHCP IPv6 tests

Actual results:
---
Traceback (most recent call last):
  File "/home/stack/tempest/tempest/api/network/test_dhcp_ipv6.py", line 378, in tearDown
    self._clean_network()
  File "/home/stack/tempest/tempest/api/network/test_dhcp_ipv6.py", line 81, in _clean_network
    self.subnets_client.delete_subnet(subnet['id'])
  File "/home/stack/tempest/tempest/lib/services/network/subnets_client.py", line 52, in delete_subnet
    return self.delete_resource(uri)
  File "/home/stack/tempest/tempest/lib/services/network/base.py", line 41, in delete_resource
    resp, body = self.delete(req_uri)
  File "/home/stack/tempest/tempest/lib/common/rest_client.py", line 307, in delete
    return self.request('DELETE', url, extra_headers, headers, body)
  File "/home/stack/tempest/tempest/lib/common/rest_client.py", line 665, in request
    resp, resp_body)
  File "/home/stack/tempest/tempest/lib/common/rest_client.py", line 778, in _error_checker
    raise exceptions.Conflict(resp_body, resp=resp)
tempest.lib.exceptions.Conflict: An object with that identifier already exists
Details: {u'message': u'Unable to complete operation on subnet 21dfe8c4-ca26-48c8-a621-8e4b1fe099e7: One or more ports have an IP allocation from this subnet.', u'type': u'SubnetInUse', u'detail': u''}
---

Expected results: Tests passed successfully

Additional info: Link to Jenkins job with the test results provided in the next comment