[SPARK-4990][Deploy]to find default properties file, search SPARK_CONF_DIR first

https://issues.apache.org/jira/browse/SPARK-4990
If user didn't pass `--properties-file` and define SPARK_CONF_DIR while not SPARK_HOME, then spark-submit will see if `spark.driver.extra*` in directory specified by `DEFAULT_PROPERTIES_FILE="$SPARK_HOME/conf/spark-defaults.conf"`. Obviously it will make the wrong judgement when SPARK_CONF_DIR does not equal `$SPARK_HOME/conf`.