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

https://issues.apache.org/jira/browse/SPARK-4990
I mean there is a possibility that user don't use the conf sub-directory default installation location but specified.
For instance, I touch a spark-defaults.conf under `/etc/my-spark/` and use it for submitting applications, so I set SPARK_CONF_DIR to `/etc/my-spark/` to make the properties file work.
The properties file under `$SPARK_HOME/conf` could be unused or used for submitting other applications.