Added FatTree simulator, extended NullProviders.java and cli null control command

Change-Id: Ibfae78306fa8663af13770abe51d202453dd1474
diff --git a/providers/null/src/main/java/org/onosproject/provider/nil/cli/NullControlCommand.java b/providers/null/src/main/java/org/onosproject/provider/nil/cli/NullControlCommand.java
index 74fbbb1..6edf6e5 100644
--- a/providers/null/src/main/java/org/onosproject/provider/nil/cli/NullControlCommand.java
+++ b/providers/null/src/main/java/org/onosproject/provider/nil/cli/NullControlCommand.java
@@ -35,7 +35,8 @@
     String cmd = null;
 
     @Argument(index = 1, name = "topoShape",
-            description = "Topology shape: e.g. configured, linear, reroute, centipede, tree, spineleaf, mesh",
+            description = "Topology shape: e.g. configured, linear, reroute, centipede, tree, spineleaf, " +
+                    ", mesh, fattree",
             required = false, multiValued = false)
     String topoShape = null;