@Override
  public List<String> getInstancesInCluster(String clusterName) {
    return new ArrayList<>(instanceNameToInstanceConfigs.keySet());
  }
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