SPARK-1492. Update Spark YARN docs to use spark-submit

Author: Sandy Ryza <sandy@cloudera.com>

Closes #601 from sryza/sandy-spark-1492 and squashes the following commits:

5df1634 [Sandy Ryza] Address additional comments from Patrick.
be46d1f [Sandy Ryza] Address feedback from Marcelo and Patrick
867a3ea [Sandy Ryza] SPARK-1492. Update Spark YARN docs to use spark-submit