Fix: do not install default rules in gateway node

1. do not proceed without selectedGatewayNode having valid value

Change-Id: Ib092ab69360f7d985c5e52d9001a8e5f446cf6dc
diff --git a/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java b/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java
index 9afae59..d6b0adc 100644
--- a/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java
+++ b/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java
@@ -753,6 +753,11 @@
      * default ARP rule to handle ARP request.
      */
     private class InternalNodeEventListener implements OpenstackNodeListener {
+        @Override
+        public boolean isRelevant(OpenstackNodeEvent event) {
+            return event.subject().type() == COMPUTE;
+        }
+
         private boolean isRelevantHelper() {
             return Objects.equals(localNodeId, leadershipService.getLeader(appId.name()));
         }