public void setClusterIdInConf(HiveConf hiveConf) {
    hiveConf.set(Constants.CLUSTER_ID_HIVE_CONF_PROP, clusterId);
  }
Logic for finding cluster id if any. Default value is empty string instead of null to stay safe
with HiveConf.set().
Precedence order: cli opt, env var, empty string
Can be used as a utility.

@return cluster id found from environment of system props
ql/src/java/org/apache/hadoop/hive/ql/ServiceContext.java