Adding commands for managing layouts.

Adding support for wiping out regions and layouts.
Refactored UI layout manager to allow public knowledge of default layout id.
Enhanced test topo for layout hierarchy.

Change-Id: I254fd494175e753f542f12a664618c1a83f72127
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopoLayoutId.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopoLayoutId.java
index 2fa8717..289cacd 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopoLayoutId.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopoLayoutId.java
@@ -23,6 +23,11 @@
  */
 public final class UiTopoLayoutId extends Identifier<String> {
 
+    /**
+     * Default topology layout identifier.
+     */
+    public static final UiTopoLayoutId DEFAULT_ID = UiTopoLayoutId.layoutId("_default_");
+
     // For serialization
     private UiTopoLayoutId() {
     }