(pup-1732) Report only services from systemd

Prior to this change the systemd service provider would return
all systemd-managed types (including sockets, devices, swap, etc).
This produced a huge list of instances, many of which were not
in fact services.

This change simply adds the '--type service' option to the
'systemctl' invocation in .instances. It also updates the
spec tests with fixtures for the old (without that option)
and new (with that option) way of invoking 'systemctl'.

I purposefully did not touch the existing fixture which is
tied to a test which is pending on a what-to-do about failed
services. If/when we tackle that, we may want to collapse
some of these fixtures.