Adding a grid topology simulator to the null provider.

Change-Id: Ie655840febc11d3986f1c8ec5c8ef2374014794c
diff --git a/providers/null/src/main/java/org/onosproject/provider/nil/CentipedeTopologySimulator.java b/providers/null/src/main/java/org/onosproject/provider/nil/CentipedeTopologySimulator.java
index 5234d44..877f7f6 100644
--- a/providers/null/src/main/java/org/onosproject/provider/nil/CentipedeTopologySimulator.java
+++ b/providers/null/src/main/java/org/onosproject/provider/nil/CentipedeTopologySimulator.java
@@ -20,9 +20,7 @@
  */
 public class CentipedeTopologySimulator extends LinearTopologySimulator {
 
-    /**
-     * Creates simulated hosts.
-     */
+    @Override
     protected void createHosts() {
         deviceIds.forEach(id -> createHosts(id, infrastructurePorts));
     }