[SPARK-2098] All Spark processes should support spark-defaults.conf, config file

This is another implementation about #1256
cc @andrewor14 @vanzin
eg:

```shell
export  SPARK_CONF_DIR=/opt/spark/config
export  SPARK_HOME=/opt/spark/spark_1.10
```

`/opt/spark/config/spark-defaults.conf` does not exist.
`/opt/spark/spark_1.10/conf/spark-defaults.conf` exists.

The above code can't load `/opt/spark/spark_1.10/conf/spark-defaults.conf`