systemd unit files should be written to /etc/systemd, not /lib/systemd

In glasklar/services/sigsum-logs#31 we're experiencing an issue where a unit file under /etc/systemd shadows the file the Ansible role has written under /lib/systemd (/lib == /usr/lib btw).

systemd.unit(5) says that /etc/systemd/system is for System units created by the administrator and that /lib/systemd/system is for System units installed by the distribution package manager so we're doing this wrong and should change.