[SPARK-7217] [STREAMING] Add configuration to control the default behavior of StreamingContext.stop() implicitly calling SparkContext.stop()

In environments like notebooks, the SparkContext is managed by the underlying infrastructure and it is expected that the SparkContext will not be stopped. However, StreamingContext.stop() calls SparkContext.stop() as a non-intuitive side-effect. This PR adds a configuration in SparkConf that sets the default StreamingContext stop behavior. It should be such that the existing behavior does not change for existing users.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #5929 from tdas/SPARK-7217 and squashes the following commits:

869a763 [Tathagata Das] Changed implementation.
685fe00 [Tathagata Das] Added configuration