[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/scalablegateway/src/main/java/org/onosproject/scalablegateway/api/ScalableGatewayService.java b/apps/scalablegateway/src/main/java/org/onosproject/scalablegateway/api/ScalableGatewayService.java
index bd1b29e..d3d0f86 100644
--- a/apps/scalablegateway/src/main/java/org/onosproject/scalablegateway/api/ScalableGatewayService.java
+++ b/apps/scalablegateway/src/main/java/org/onosproject/scalablegateway/api/ScalableGatewayService.java
@@ -35,12 +35,12 @@
     GatewayNode getGatewayNode(DeviceId deviceId);
 
     /**
-     * Returns the list of gateway`s port numbers with the given device identifier.
+     * Returns the gateway`s port number with the given device identifier.
      *
      * @param deviceId The gateway node deviceId
-     * @return The list of external interface port number
+     * @return The external interface port number
      */
-    List<PortNumber> getGatewayExternalPorts(DeviceId deviceId);
+    PortNumber getGatewayExternalPort(DeviceId deviceId);
 
     /**
      * Returns group id for gateway load balance.