SPARK-1746: Support setting SPARK_JAVA_OPTS on executors for backwards compatibility

Author: Patrick Wendell <pwendell@gmail.com>

Closes #676 from pwendell/worker-opts and squashes the following commits:

54456c4 [Patrick Wendell] SPARK-1746: Support setting SPARK_JAVA_OPTS on executors for backwards compatibility
(cherry picked from commit 913a0a9c0a87e164723ebf9616b883b6329bac71)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>