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/NullProviders.java b/providers/null/src/main/java/org/onosproject/provider/nil/NullProviders.java
index b0ccf15..5d65caf 100644
--- a/providers/null/src/main/java/org/onosproject/provider/nil/NullProviders.java
+++ b/providers/null/src/main/java/org/onosproject/provider/nil/NullProviders.java
@@ -365,6 +365,8 @@
             return new MeshTopologySimulator();
         } else if (topoShape.matches("grid([,].*|$)")) {
             return new GridTopologySimulator();
+        } else if (topoShape.matches("fattree([,].*|$)")) {
+            return new FatTreeTopologySimulator();
         } else if (topoShape.matches("custom([,].*|$)")) {
             return new CustomTopologySimulator();
         } else {