Fix: obtain the correct external gateway NAT IP address

1. Add verbose message in stateful SNAT rules installation phase
2. Refactor openstack networking util

Change-Id: Ia74a529657bf0e7b34957053e1411786ba3fe0d5
diff --git a/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackRoutingArpHandler.java b/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackRoutingArpHandler.java
index b6bde75..04e6b62 100644
--- a/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackRoutingArpHandler.java
+++ b/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackRoutingArpHandler.java
@@ -96,9 +96,9 @@
 import static org.onosproject.openstacknetworking.impl.OsgiPropertyConstants.ARP_MODE_DEFAULT;
 import static org.onosproject.openstacknetworking.impl.OsgiPropertyConstants.GATEWAY_MAC_DEFAULT;
 import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.associatedFloatingIp;
+import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.externalGatewayIp;
 import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.externalPeerRouterForNetwork;
 import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.floatingIpByInstancePort;
-import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.getExternalIp;
 import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.getGwByComputeDevId;
 import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.getGwByInstancePort;
 import static org.onosproject.openstacknetworking.util.OpenstackNetworkingUtil.getPropertyValue;
@@ -539,7 +539,7 @@
 
     private void setFakeGatewayArpRuleByRouter(Router router, boolean install) {
         if (ARP_BROADCAST_MODE.equals(getArpMode())) {
-            IpAddress externalIp = getExternalIp(router, osNetworkService);
+            IpAddress externalIp = externalGatewayIp(router, osNetworkService);
 
             if (externalIp == null) {
                 log.debug("External IP is not found");