Fix #93: Add description to list units

Get the unit description from the systemd unit to show it in the unit listing.
As an alternative implementation, the description could be added as a field to job.Job and job.Payload.

:warning: I have not `fleet` running anywhere yet, so I haven't really tested this. I haven't seen any test for `fleetctl` in the repo either, so I don't know what's your convention about testing the ctl.