protected JSONObject makeJsonPartitionLayout() throws JSONException {
      return makeJsonPartitionLayout(DEFAULT_PARTITION_CLASS);
    }
Get given number of datanodes from each datacenter.
@param dataNodeCountPerDc number of datanodes to get from each datacenter.
@return a list of datanodes.
ambry-clustermap/src/test/java/com/github/ambry/clustermap/TestUtils.java