[ONOS-4871] Fix gateway external port from list to single port

 - Fix GatewayNode externalInterface from list to string
 - Fix GatewayService externalPort from list to portNumber

Change-Id: I8869c7bf550e005db854b464763cc2bc321faa6a
diff --git a/apps/openstacknetworking/routing/src/main/java/org/onosproject/openstacknetworking/routing/OpenstackIcmpHandler.java b/apps/openstacknetworking/routing/src/main/java/org/onosproject/openstacknetworking/routing/OpenstackIcmpHandler.java
index d9e632b..6eaddf1 100644
--- a/apps/openstacknetworking/routing/src/main/java/org/onosproject/openstacknetworking/routing/OpenstackIcmpHandler.java
+++ b/apps/openstacknetworking/routing/src/main/java/org/onosproject/openstacknetworking/routing/OpenstackIcmpHandler.java
@@ -331,7 +331,7 @@
     private Map<DeviceId, PortNumber> getExternalInfo() {
         Map<DeviceId, PortNumber> externalInfoMap = Maps.newHashMap();
         gatewayService.getGatewayDeviceIds().forEach(deviceId ->
-                externalInfoMap.putIfAbsent(deviceId, gatewayService.getGatewayExternalPorts(deviceId).get(0)));
+                externalInfoMap.putIfAbsent(deviceId, gatewayService.getGatewayExternalPort(deviceId)));
         return externalInfoMap;
     }
 }