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

This is another implementation about #1256
cc @andrewor14 @vanzin
+1 on factoring it out into `Utils`. These few lines are identical especially across `Master` and `Worker`. Though maybe I would call it something else, like `loadDefaultSparkProperties` (the "default" here is important because we call `setIfMissing` instead of `set`, and `getOrElseUpdate` instead of just overriding the system property).