[SPARK-1749] Job cancellation when SchedulerBackend does not implement killTask

It turns out that having the DAGScheduler tell the taskScheduler to cancelTasks when the backend does not implement killTask (e.g. Mesos) is not such a good idea.
Ok, either tonight or tomorrow I can update this PR to reflect that strategy, or you can go ahead and make the change @pwendell.  Outside the immediate scope of this PR, what prevents Mesos from being able to kill tasks, and when do we expect that to change?