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

This is another implementation about #1256
cc @andrewor14 @vanzin
This block is repeated 3 times, maybe add it to Utils? Also, wonder if this would look better:

    Option(arg.propertiesFile)
      .map { f => Utils.getPropertiesFromFile(f) }
      .filter { case (k, v) => k.startsWith("spark.") }
      .foreach { case (k, v) => sys.props.getOrElseUpdate(k, v) }