add get description in VpcGateway

Author: charlesweng

api/src/main/java/com/cloud/network/vpc/VpcGateway.java

@@ -64,6 +64,11 @@ public enum State {
      */
     String getNetmask();
 
+    /**
+     * @return
+     */
+    String getDescription();
+
     /**
      * @return
      */