Add a small enhancement to generate a key string for a node port tuple

src/main/java/net/floodlightcontroller/topology/NodePortTuple.java

@@ -84,4 +84,8 @@ public class NodePortTuple {
             return false;
         return true;
     }
+    
+    public String toKeyString() {
+        return (HexString.toHexString(nodeId)+ "|" + portId);
+    }
 }