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

This is another implementation about #1256
cc @andrewor14 @vanzin
I don't want to put too many things in a Utils's method. The method should be kept simple.
How about this?
```scala
  def loadSparkProperties(properties: Map[String, String],
    conf: SparkConf): Unit = {
    properties.filter { case (k, v) =>
      k.startsWith("spark.")
    }.foreach { case (k, v) =>
      conf.setIfMissing(k, v)
      sys.props.getOrElseUpdate(k, v)
    }
  }
```