Fix /var unmount error during shutdown

The problem with failed /var unmount placed on separate partition have a long story: https://github.com/systemd/systemd/issues/867 According messages after the fix integrated into upstream -- there are some corner cases which aren't fully fixed by patches adding relinquish options

In case of ostree-based images we have a similar issue since /var is bind-mounted in initramfs. Systemd is trying to unmount /var on shutdown, however ExecStop= command from systemd-journal-flush service is not executed during shutdown. By adding PartOf=var.mount into service file we force the systemd-journal-flush.service to be called prior the /var unmount. This allow to unlock the bind-mount, since journald have a chance to re-link it's journal into /run with journalctl --smart-relinquish-var call.