Validate systemd unit files (#10597)

* Validate systemd unit files

This ensure that we fail early if we have a bad systemd unit file
(syntax error, using a version not available in the local version, etc)

* Hack to check systemd version for service files validation

factory-reset.target was introduced in system 250, same version as the
aliasing feature we need for verifying systemd services with ansible.
So we only actually executes the validation if that target is present.

This is an horrible hack which should be reverted as soon as we drop
support for distributions with systemd<250.