feat(corectl): Expose job state in list-units cmd
refactor(corectl): Minimize calls to etcd in list-units cmd

Rather than make one query per machine to list jobs, we can just
list all jobs and pull the machine from the JobState that is
reported by the owner of the Job.
Merge pull request #19 from bcwaldon/corectl-list-improvements

Improve corectl list-units - more data & fewer queries to etcd!