REQUEST: provide a callback for success/failure

[spil-paoloiannelli]

Based on the result of the job, we should be able to pass to job-runner a callback function that will be triggered based on the result of the job.

IE. if the job fails we want to provide a normal python function as argument to clean up the dirty environment that could have been left from the failed job.
On the other hand, in case a job succeed, we could hook a specific function to execute.

Generically, it should be possible to attach a callback that will be triggered based on job status change.