Set ML2/OVS backend explicitly in the devstack jobs

Neutron team recently switched default backend used in Neutron
by Devstack to OVN. With that backend some tests, like e.g. related
to DHCP or L3 agents aren't working fine. So to have still the same
test coverage as we had before, let's explicitly set ML2/OVS as a
Neutron's backend in those CI jobs.

Change-Id: Idf6466a59c6cf96be2f1d53e696f0564584fa233

.zuul.yaml

@@ -88,11 +88,28 @@
         # NOTE(amotoki): Some neutron features are enabled by devstack plugin
         neutron: https://opendev.org/openstack/neutron
       devstack_services:
+        # 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
+      devstack_localrc:
+        Q_AGENT: openvswitch
+        Q_ML2_TENANT_NETWORK_TYPE: vxlan
+        Q_ML2_PLUGIN_MECHANISM_DRIVERS: openvswitch
       tox_envlist: functional
 
 - job: