List<PartitionId> getPartitions(String partitionClass) {
      return getPartitionsInClass(partitionClass, false);
    }
Gets all partitions belonging to the {@code paritionClass} (all partitions if it is {@code null}).
@param partitionClass the class of the partitions desired. Can be {@code null}.
@return all the partitions in {@code partitionClass} (all partitions if it is {@code null})
ambry-clustermap/src/main/java/com/github/ambry/clustermap/ClusterMapUtils.java