supervisor.exec_config refactor - no tests (review)

`supervisor.execute_launch_config` returned `Deferred` that fired back with job id and completion deferred. The job id was used by `_Job`. Now, it only returns completion and job id knowledge moved to `_Job`. This way code is simpler. This is based on discussion in https://github.com/rackerlabs/otter/pull/646#discussion-diff-13937435. Unfortunately, most of the discussion happened offline. 
Also, I was thinking of this to be only refactor and do the pool addition in #646. I can do it one PR if you prefer that.