Several fixes

Use an etcd Create rather than a Set when creating Jobs, so we can tell if the job previously exists internally when deciding whether to create job offers or not. This will now prevent someone from calling 'fleetctl start' on the same job twice in a row without calling 'stop' first

The 'fleetctl stop' operation was only removing part of the Job's data from etcd, leaving the original job object around to prevent future jobs with the same name from being created.

Even though the known issue was fixed, the list-units command now uses the list of known payloads *and* orphaned jobs to build its output.