[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


Discussed in person and we decided that this is fine.