In particular when the HADOOP_CONF_DIR is not not specified.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #488 from pwendell/hadoop-cleanup and squashes the following commits:

fe95f13 [Patrick Wendell] Changes based on Andrew's feeback
18d09c1 [Patrick Wendell] Review comments from Andrew
17929cc [Patrick Wendell] Assorted clean-up for Spark-on-YARN.