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

This is another implementation about #1256
cc @andrewor14 @vanzin
I know I wrote this in the bug, but I'm wondering if this is really OK. Before, except for spark-submit, no daemons would read the default config file. So this has the potential to add config data that might make things behave differently when running the new Spark.

Just a thought, though. I think the new behavior makes more sense, but it might trip some people.