@Override
  public boolean addCluster(String clusterName) {
    if (this.clusterName == null) {
      this.clusterName = clusterName;
      return true;
    } else {
      throw new IllegalStateException("A cluster has already been added");
    }
  }
Get the instances that have replicas for the given partition.
@param partition the partition name of the partition.
@return the set of instances that have replicas for this partition.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/MockHelixAdmin.java