Add `engine` functionality to disable the job supervisor

Also needed for https://bugs.launchpad.net/oq-engine/+bug/1206913.

Basically, in the oq-engine-server we are running the job "control node"/main process in a celeryd worker. If we run with supervision, we end up forking the process twice to enable a supervisor process. This is bad for the oq-engine-server because this results in leaked celeryd processes. (We are using this feature here: https://github.com/gem/oq-engine-server/pull/7/files#L1R41.)

https://github.com/gem/oq-engine/pull/1255 should merge first for a clean diff.