This is provoked by de91fe933d,
where I've added a few new labels and made it possible for people to
disable them.
In this patch, I'm making it possible to disable any of the old Traefik
labels in a similar way.
This moves the comments from being just in Jinja,
to actually ending up in the generated `labels` file,
which makes inspection of the final result easier.
Also, some new lines were added here and there to make labels
more legible.
The generated file may still include weird new-lines due to
various `if` statements yielding content or not, but that's not so ugly
anymore - now that we have proper start/end sections that are visible in
the final `labels` file.
