DVR-Enable ARP Responder when DVR and L2pop is enabled

This patch enables ARP Responder with DVR routers along with
l2pop in devstack.

Related-Bug: #1774459

Change-Id: I82f628c32f6e38c2419b6ffe90d9f9adf96777b1
diff --git a/lib/neutron b/lib/neutron
index 1066d8e..17ffce7 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -237,6 +237,7 @@
             if [[ "$NEUTRON_DISTRIBUTED_ROUTING" = "True" ]]; then
                 iniset $NEUTRON_CORE_PLUGIN_CONF agent l2_population True
                 iniset $NEUTRON_CORE_PLUGIN_CONF agent enable_distributed_routing True
+                iniset $NEUTRON_CORE_PLUGIN_CONF agent arp_responder True
             fi
         fi