Added ui-cache-regions CLI command.

Change-Id: I196543792c85e40e582f9456e4e50dd33bedf111
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
index dd601eb..b4eabe9 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
@@ -149,6 +149,16 @@
         return cnodeLookup.size();
     }
 
+
+    /**
+     * Returns all regions in the model.
+     *
+     * @return all regions
+     */
+    public Set<UiRegion> allRegions() {
+        return new HashSet<>(regionLookup.values());
+    }
+
     /**
      * Returns a reference to the null-region. That is, the container for
      * devices, hosts, and links that belong to no region.