SPARK-1588.  Restore SPARK_YARN_USER_ENV and SPARK_JAVA_OPTS for YARN.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #586 from sryza/sandy-spark-1588 and squashes the following commits:

35eb38e [Sandy Ryza] Scalify
b361684 [Sandy Ryza] SPARK-1588.  Restore SPARK_YARN_USER_ENV and SPARK_JAVA_OPTS for YARN.