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

This is another implementation about #1256
cc @andrewor14 @vanzin
That makes the call sites too complicated for my taste (since all of them need to deal with "what's the default config file" and "how to load properties into a map"), but no strong opinion. 

Since this method is already so single-purpose (load properties into an existing SparkConf), I feel like my suggestion makes things cleaner.