neutronclient.tests.functional.core.test_readonly_neutron.SimpleReadOnlyNeutronClientTest.test_neutron_dhcp_agent_list_hosting_net repeatedly fails

Currently the neutronclient-functional job always fails because of the following test case.

```
neutronclient.tests.functional.core.test_readonly_neutron.SimpleReadOnlyNeutronClientTest.test_neutron_dhcp_agent_list_hosting_net
----------------------------------------------------------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/home/zuul/src/opendev.org/openstack/python-neutronclient/neutronclient/tests/functional/core/test_readonly_neutron.py", line 55, in test_neutron_dhcp_agent_list_hosting_net
    self.neutron('dhcp-agent-list-hosting-net',

      File "/home/zuul/src/opendev.org/openstack/python-neutronclient/neutronclient/tests/functional/base.py", line 69, in neutron
    return self.clients.neutron(*args, **kwargs)

      File "/home/zuul/src/opendev.org/openstack/python-neutronclient/.tox/functional/lib/python3.8/site-packages/tempest/lib/cli/base.py", line 311, in neutron
    return self.cmd_with_auth(

      File "/home/zuul/src/opendev.org/openstack/python-neutronclient/.tox/functional/lib/python3.8/site-packages/tempest/lib/cli/base.py", line 394, in cmd_with_auth
    return execute(cmd, action, flags, params, fail_ok, merge_stderr,

      File "/home/zuul/src/opendev.org/openstack/python-neutronclient/.tox/functional/lib/python3.8/site-packages/tempest/lib/cli/base.py", line 63, in execute
    raise exceptions.CommandFailed(proc.returncode,

    tempest.lib.exceptions.CommandFailed: Command '['/home/zuul/src/opendev.org/openstack/python-neutronclient/.tox/functional/bin/neutron', '--os-username', 'admin', '--os-project-name', 'admin', '--os-password', 'secretadmin', '--os-auth-url', 'http://158.69.75.185/identity', '--os-user-domain-id', 'default', '--os-project-domain-id', 'default', '--endpoint-type', 'publicURL', 'dhcp-agent-list-hosting-net', 'private']' returned non-zero exit status 1.
stdout:
b''
stderr:
b"neutron CLI is deprecated and will be removed in the Z cycle. Use openstack CLI instead.\nThe resource could not be found.\nNeutron server returns request_ids: ['req-fb801512-842c-42ab-8292-936f76f1ee1d']\n"
```

An example can be found here:
https://zuul.opendev.org/t/openstack/build/c587b6812cc14c55b08c62d414752106

This is likely to be an failure caused by recent switch from ovs to ovn.