[SPARK-1112, 2156] Bootstrap to fetch the driver's Spark properties.

This is an alternative solution to #1124 . Before launching the executor backend, we first fetch driver's spark properties and use it to overwrite executor's spark properties. This should be better than #1124.

@pwendell Are there spark properties that might be different on the driver and on the executors?
This comment is no longer technically true -- I think we do have all the information to start a proper SparkEnv, whose actor system we could use instead. Also, RegisteredExecutor() no longer needs t otake these properties.