[2.6] Added support for Service Discovery in Spark

This adds Service Discovery in Spark. 

The ServiceDiscovery code has just been copied and pasted to extract it out from AbstractContext so that we can serialize it to pass to Spark worker.



@anew: Please review. Thanks.

Build running here: https://builds.cask.co/browse/CDAP-DUT109-3


Added the null part in docs. 
I am not sure what better function doc description can be. @ParthGandhi : Since you wrote this can you suggest something ?