Spark Integration: static BasicSparkContext rather than from injector

This changes how we get hold of the BasicSparkContext outside CDAP in a Spark program. Earlier we used to create it back from conf and use the injector now its becomes a static for reuse.

Build running here: https://builds.cask.co/browse/CDAP-DUT177-2

@anew : Please review. Thanks.
I don't like a "pull" model like this, as this mess-up with class relationship. Why not have the BasicSparkContext pass-in through the constructor?