Provide a way to "kill" a task but mark it as successful

[wyegelwel]

There are frameworks that will submit jobs to cook that are "workers" and then kill the worker when it is no longer needed. In cook this is registered as a failure but the framework is simply shutting down the worker.

We should provide a way to mark a task in this case killed but successful so that the framework and cook can have better reporting.

[dposada]

Should this just be a flag (e.g. mark-success=true) on the kill endpoints (DELETE /rawscheduler, DELETE /group)?