dall back to HADOOP_CONF_DIR when YARN_CONF_DIR not specified
dall back to HADOOP_CONF_DIR when YARN_CONF_DIR not specified

kevinykuo/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,rstudio/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr