SPARK-1639. Tidy up some Spark on YARN code

This contains a bunch of small tidyings of the Spark on YARN code.

I focused on the yarn stable code.  @tgravescs, let me know if you'd like me to make these for the alpha code as well.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #561 from sryza/sandy-spark-1639 and squashes the following commits:

72b6a02 [Sandy Ryza] Fix comment and set name on driver thread
c2190b2 [Sandy Ryza] SPARK-1639. Tidy up some Spark on YARN code