Fix errors in designate scenarios jobs

Even though we do not test any actual port bindings, the lack of a
properly configured agent leads to errors in the server logs that may
cause confusion when trying to debug real issues.

So copy the setup from the openvswitch scenario jobs, which is the most
simple one.

Change-Id: I050aacbc1bacbadbd94359c1ab8f1ac721b90a12

.zuul.yaml

@@ -785,9 +785,24 @@
         DESIGNATE_BACKEND_DRIVER: bind9
         DOWNLOAD_DEFAULT_IMAGES: false
         IMAGE_URLS: http://download.cirros-cloud.net/0.3.4/cirros-0.3.4-i386-disk.img,
+        Q_AGENT: openvswitch
         # In this job advanced image is not needed, so it's name should be
         # empty
         ADVANCED_IMAGE_NAME: ""
+      devstack_local_conf:
+        post-config:
+          $NEUTRON_CONF:
+            DEFAULT:
+              enable_dvr: false
+          # NOTE(slaweq): We can get rid of this hardcoded absolute path when
+          # devstack-tempest job will be switched to use lib/neutron instead of
+          # lib/neutron-legacy
+          /$NEUTRON_CORE_PLUGIN_CONF:
+            agent:
+              tunnel_types: vxlan,gre
+            ovs:
+              tunnel_bridge: br-tun
+              bridge_mappings: public:br-ex
       devstack_plugins:
         designate: https://opendev.org/openstack/designate.git
       devstack_services: