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

This is another implementation about #1256
cc @andrewor14 @vanzin
是这样的:在spark的源码中有非常多这样的代码:` val conf = new SparkConf` 例如:
[SparkHadoopUtil.scala#L38](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/SparkHadoopUtil.scala#L38) ,[EventLoggingListener.scala#L216](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/scheduler/EventLoggingListener.scala#L216),
这要求我们必须要设置`system property `. 

这样的话如果不改变初始话顺序, 我们需要同时设置`conf` 和 `sys.props`.