[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?

Author: Xiangrui Meng <meng@databricks.com>

Closes #1132 from mengxr/akka-bootstrap and squashes the following commits:

77ff32d [Xiangrui Meng] organize imports
68e1dfb [Xiangrui Meng] use timeout from AkkaUtils; remove props from RegisteredExecutor
46d332d [Xiangrui Meng] fix a test
7947c18 [Xiangrui Meng] increase slack size for akka
4ab696a [Xiangrui Meng] bootstrap to retrieve driver spark conf