Spark Integration: MapReduceContext extends ServiceDiscoverer

MapReduceConext should extend ServiceDiscoverer like ProcedureContext and FlowletContext to support ServiceDiscovery in beforeSubmit() of MapReduce which has MapReduceContext else the user cannot do ServiceDiscovery in MapReduce at all.

This passed through initially as there is no app, unit test or usage of service discover in MapReduce. Have opened a Jira issue for it here: https://issues.cask.co/browse/CDAP-654