SPARK-1713. Use a thread pool for launching executors.

This patch copies the approach used in the MapReduce application master for launching containers.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #663 from sryza/sandy-spark-1713 and squashes the following commits:

036550d [Sandy Ryza] SPARK-1713. [YARN] Use a threadpool for launching executor containers