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

https://issues.apache.org/jira/browse/SPARK-4990
OK I see the scenario now where this matters. So the default config for an installation might in fact set these special properties, which the script needs to handle before `SparkSubmit` starts. Maybe someone else can double-check, but that makes sense to me.