### Scenario ###
1) Deploy Overcloud without DNS Neutron Plugin Extension.
Do not include the template needed: virt/dns.yaml with the following content:
parameter_defaults:
    ExtraConfig:
        neutron::dns_domain: openstackgate.local
    NeutronPluginExtensions: qos,port_security,dns

2) Install neutron-tempest-plugin and run: “test_create_update_port_with_dns_name” with:
python3 -m testtools.run neutron_tempest_plugin.api.test_ports.PortsTestJSON.test_create_update_port_with_dns_name


### Actual Result ###
1) Tempest test fails(as expected) with:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-
    return func(*func_args, **func_kwargs)
  File "/home/stack/neutron-tempest-
    self.assertEqual('d1', body['dns_name'])
  File "/usr/lib/python3.6/site-
    self.assertThat(observed, matcher, message)
  File "/usr/lib/python3.6/site-
    raise mismatch_error
testtools.matchers._impl.

2) It looks like a given "dns_name" is simply ignored by Neutron and the reason for that is unknown, there is nothing logged on that in neutron logs.

3) API response code is 201 and no error in response body. 
According to API response payload, “dns_name” is actually empty
2020-11-01 10:37:46,251 943702 DEBUG    [tempest.lib.common.rest_
        Body: {"port": {"network_id": "6859d4d1-429a-4b78-8d6e- "dns_name": "d1"}}
    Response - Headers: {'content-type': 'application/json', 'content-length': '877', 'x-openstack-request-id': 'req-f50a1e45-6d93-41f0-aaf8-http://10.0.0.120:9696/v2.0/'}
        Body: b'{"port":{"id":"b1e03f0f-dns_name":"",
}}}


### Expected Result ###
When DNS extension is not set/configured/installed we would expect that the API will fail with 4XX status code + appropriate Error message in response body.