Tempest Rhel7: slaac subnet cant be deleted with attached port

Build: R3.0/51/RHEL/LIBERTY

According to testcase: test_create_delete_slaac_subnet_with_ports
https://github.com/openstack/tempest/blob/master/tempest/api/network/test_networks.py#L637

a slaac subnet with attached port can be deleted. However its not allowed in the current build.

Test Desc:
        Create subnet with SLAAC, create ports in network
        and then you shall be able to delete subnet without port
        deletion. But you still can not delete the network.

Traceback (most recent call last):
  File "tempest/api/network/test_networks.py", line 638, in test_create_delete_slaac_subnet_with_ports
    self._test_delete_subnet_with_ports("slaac")
  File "tempest/api/network/test_networks.py", line 618, in _test_delete_subnet_with_ports
    self.subnets_client.delete_subnet(subnet_slaac['id'])
  File "tempest/lib/services/network/subnets_client.py", line 34, 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 290, in delete
    return self.request('DELETE', url, extra_headers, headers, body)
  File "tempest/lib/common/rest_client.py", line 641, in request
    resp, resp_body)
  File "tempest/lib/common/rest_client.py", line 699, in _error_checker
    raise exceptions.BadRequest(resp_body, resp=resp)
tempest.lib.exceptions.BadRequest: Bad request
Details: {u'message': u'Bad network request: Cannot Delete IP Block, Instance IP(s) in use', u'type': u'BadRequest', u'detail': u''}