Set ML2/OVS backend explicitly for functional job

After default backend change to OVN, some client tests related to DHCP
or L3 agents are not working fine. Switch the job to ML2/OVS, as it was
done in openstack-client in changeset
Idf6466a59c6cf96be2f1d53e696f0564584fa233

test_neutron_dhcp_agent_list_hosting_net was skipped temporarily
to pass the gate in the parent commit. It is re-enabled now.

Change-Id: I91d03d13adcd38d46ba1b042239a290ae2c99ea4
Closes-Bug: #1936965

.zuul.yaml

@@ -38,9 +38,31 @@
         # NOTE: neutronclient.tests.functional.base.ClientTestBase does not
         # support HTTPS endpoints now, so tls-proxy needs to be disabled.
         tls-proxy: false
+        # Disable OVN services
+        br-ex-tcpdump: false
+        br-int-flows: false
+        ovn-controller: false
+        ovn-northd: false
+        ovs-vswitchd: false
+        ovsdb-server: false
+        q-ovn-metadata-agent: false
+        # Neutron services
+        q-agt: true
+        q-dhcp: true
+        q-l3: true
+        q-meta: true
+        neutron-network-segment-range: true
+        neutron-segments: true
+        q-metering: true
+        q-qos: true
+        neutron-tag-ports-during-bulk-creation: true
+        neutron-conntrack-helper: true
       devstack_localrc:
         USE_PYTHON3: true
         LIBS_FROM_GIT: python-neutronclient
+        Q_AGENT: openvswitch
+        Q_ML2_TENANT_NETWORK_TYPE: vxlan
+        Q_ML2_PLUGIN_MECHANISM_DRIVERS: openvswitch
       devstack_plugins:
         neutron-vpnaas: https://opendev.org/openstack/neutron-vpnaas
 

neutronclient/tests/functional/core/test_readonly_neutron.py

@@ -13,7 +13,6 @@
 import re
 
 from tempest.lib import exceptions
-import testtools
 
 from neutronclient.tests.functional import base
 
@@ -52,7 +51,6 @@ class SimpleReadOnlyNeutronClientTest(base.ClientTestBase):
         ext = self.parser.listing(self.neutron('ext-list'))
         self.assertTableStruct(ext, ['alias', 'name'])
 
-    @testtools.skip('Skipped until ML2/OVS is enabled')
     def test_neutron_dhcp_agent_list_hosting_net(self):
         self.neutron('dhcp-agent-list-hosting-net',
                      params='private')