Bug 26363: Provide a systemd unit file for koha-worker in package installs

Bug 26363: Provide a systemd unit file for koha-worker in package installs

While the packages already have a koha-zebra script that creates a service
using the 'daemon' utility, this unit file template provides an alternative.

Note that currently you'd have to start the service using the following:

```systemctl start koha-worker@NAME.service```

Where NAME is the name of your Koha instance.

The systemctl enable/start commands could be automated more with helper
scripts in debian/scripts, but this is a first step.

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>