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_plugins/ml2 b/lib/neutron_plugins/ml2
index c5a4c02..a746683 100644
--- a/lib/neutron_plugins/ml2
+++ b/lib/neutron_plugins/ml2
@@ -147,6 +147,7 @@
         populate_ml2_config /$Q_PLUGIN_CONF_FILE agent l2_population=True
         populate_ml2_config /$Q_PLUGIN_CONF_FILE agent tunnel_types=vxlan
         populate_ml2_config /$Q_PLUGIN_CONF_FILE agent enable_distributed_routing=True
+        populate_ml2_config /$Q_PLUGIN_CONF_FILE agent arp_responder=True
     fi
 }