SPARK-1860: Do not cleanup application work/ directories by default

This causes an unrecoverable error for applications that are running for longer
than 7 days that have jars added to the SparkContext, as the jars are cleaned up
even though the application is still running.

Author: Aaron Davidson <aaron@databricks.com>

Closes #800 from aarondav/shitty-defaults and squashes the following commits:

a573fbb [Aaron Davidson] SPARK-1860: Do not cleanup application work/ directories by default