Check worker identity

[nolan1999]

Do not ask workers to provide their identity; instead, get it from the authentication.
When a status update is pushed, check that it is the latest worker registered for the job in the API: if not, send cancel signal.
Combine with ries-lab/DECODE_Cloud_UserAPI#8

[nolan1999]

Also, job status updates / files upload should only be permitted for the latest worker

[nolan1999]

Also, worker's environment should be gotten from its group (e.g. add group "cloud" for cloud workers)