ONOS-4326: Focusing on add/remove cluster member. (WIP).

If reviewing this, please refer to http://tinyurl.com/onos-ui-topo-model

Change-Id: Ic6568074ac768ec828f9103e92caab5e9a06ade6
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 a5de405..40de28c 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
@@ -28,12 +28,14 @@
  */
 class UiCluster extends UiElement {
 
+    private static final String DEFAULT_CLUSTER_ID = "CLUSTER-0";
+
     private final List<UiClusterMember> members = new ArrayList<>();
     private final Map<NodeId, UiClusterMember> lookup = new HashMap<>();
 
     @Override
     public String toString() {
-        return String.valueOf(members.size()) + "-member cluster";
+        return String.valueOf(size()) + "-member cluster";
     }
 
     /**
@@ -65,6 +67,16 @@
     }
 
     /**
+     * Removes the given member from the cluster.
+     *
+     * @param member member to remove
+     */
+    public void remove(UiClusterMember member) {
+        members.remove(member);
+        lookup.remove(member.id());
+    }
+
+    /**
      * Returns the number of members in the cluster.
      *
      * @return number of members
@@ -72,4 +84,9 @@
     public int size() {
         return members.size();
     }
+
+    @Override
+    public String idAsString() {
+        return DEFAULT_CLUSTER_ID;
+    }
 }