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

This is another implementation about #1256
cc @andrewor14 @vanzin
Hm looks like the import ordering is a little messed up now. Looks like here we want to load properties before initializing `SparkConf`, so `WorkerArguments` can no longer take in a conf. This logic seems a little strange to me.