private Cluster createCluster(String clusterName) throws AmbariException {
    StackId stackId = new StackId("HDP-0.1");

    helper.createStack(stackId);

    clusters.addCluster(clusterName, stackId);

    return clusters.getCluster(clusterName);
  }
Tests that {@link HostRegisteredEvent} properly updates the
{@link Clusters} in-memory mapping of hostIds to hosts.

@throws AmbariException
ambari-server/src/test/java/org/apache/ambari/server/state/cluster/ClustersTest.java