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

Hm, I actually don't think any of this logic should exist in `SparkConf`. The current model is that by the time we get here all the properties from the file are already loaded into `sys.props` so we can just load those. This is a nice abstraction because `SparkConf` doesn't have to be concerned with where the properties file is.