Started fleshing out the UI topo model classes.
If reviewing this, please refer to http://tinyurl.com/onos-ui-topo-model

Change-Id: I4738392bec1a89c37dff15eff6fe04d66fcabd95
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiCluster.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiCluster.java
index 87afc56b..f7879ef 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiCluster.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiCluster.java
@@ -16,8 +16,20 @@
 
 package org.onosproject.ui.model.topo;
 
+import java.util.ArrayList;
+import java.util.List;
+
 /**
  * Encapsulates the notion of the ONOS cluster.
  */
-public class UiCluster extends UiElement {
+class UiCluster extends UiElement {
+
+    private final List<UiClusterMember> members = new ArrayList<>();
+
+    /**
+     * Removes all cluster members.
+     */
+    void clear() {
+        members.clear();
+    }
 }