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

This is another implementation about #1256
cc @andrewor14 @vanzin
I think all daemon properties are pretty specific to the daemons and don't affect the execution of the application, so it seems safe to me. Once we add this we should probably also deprecate `SPARK_MASTER_OPTS` etc in favor of doing it through `spark-defaults.conf`. Then maybe we can add a line in `spark-defaults.conf.template` that gives an example of a Master config being set here or something.