DHCPv[46] relay must replace the route instead of adding an extra NH

Currently DHCP relay adds an extra next-hop to an already existing route (if any). Instead
it must replace it with the new one.

This patch adds replaceRoute method to the RouteStore interface and corresponding implementations.
The DHCP relay implementation uses the new method instead of updateRoute

Change-Id: I601613168b83290db4e53d2aae5c86e963ae17b0
diff --git a/apps/dhcprelay/src/test/java/org/onosproject/dhcprelay/DhcpRelayManagerTest.java b/apps/dhcprelay/src/test/java/org/onosproject/dhcprelay/DhcpRelayManagerTest.java
index 25f7b67..9d86606 100644
--- a/apps/dhcprelay/src/test/java/org/onosproject/dhcprelay/DhcpRelayManagerTest.java
+++ b/apps/dhcprelay/src/test/java/org/onosproject/dhcprelay/DhcpRelayManagerTest.java
@@ -930,6 +930,11 @@
         public void removeRoute(Route route) {
             routes.remove(route);
         }
+
+        public void replaceRoute(Route route) {
+            routes.remove(route);
+            routes.add(route);
+        }
     }
 
     private class MockInterfaceService extends InterfaceServiceAdapter {