SPARK-1864 Look in spark conf instead of system properties when propagating configuration to executors.

Author: Michael Armbrust <michael@databricks.com>

Closes #808 from marmbrus/confClasspath and squashes the following commits:

4c31d57 [Michael Armbrust] Look in spark conf instead of system properties when propagating configuration to executors.
(cherry picked from commit a80a6a139e729ee3f81ec4f0028e084d2d9f7e82)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>