`base::service_unit` was a handy bridge abstraction during the migration from upstart to systemd. Given that's mostly over now, let's deprecate and progressively remove it from puppet, as we have better resources to manage systemd units in `systemd::service` and `systemd::unit`
Steps:
[] Add a deprecation warning to `base::service_unit` whenever it's used without an upstart or sysvinit parameter
[] Add a wmfstyleguide violation for use of `base::service_unit`, so that any new declaration will trigger a -1 from CI
[] Convert all the systemd-only `base::service_unit` to use `systemd::service` or `systemd::unit`
[] Check and remove unused upstart scripts/declaration and convert the corresponding units
Once we got rid of trusty
[] Convert the remaining declarations
[] Remove `base::service_unit` from puppet
[] Remove the wmf styleguide check