Install systemd units in the right location

Debian uses /lib/systemd/system for system unit files, while i'm
putting ostree under the /usr prefix which means the hardcoded path
fails. Leave it to configure to work out the right location for systemd
units (method copied from pollkit).

Furthermore instead of installing the unit in local-fs.target.wants by
hand add a [Install] section so systemctl enable does the right thing

https://bugzilla.gnome.org/show_bug.cgi?id=705864