If SPARK_CONF_DIR environment variable is set, search it for spark-defaults.conf.

Author: Albert Chu <chu11@llnl.gov>

Closes #1059 from chu11/SPARK-2116 and squashes the following commits:

9f3ac94 [Albert Chu] SPARK-2116: If SPARK_CONF_DIR environment variable is set, search it for spark-defaults.conf.