systemd-shutdown is run after the network is stopped, so remounting
a network filesystem read-only can hang. A simple umount is the most
useful thing that can be done for a network filesystem once the
network is down.
For Debian 8 jessie, these problems have been fixed in version 215-17+deb8u14.
We recommend that you upgrade your systemd packages.
Further information about Extended LTS security advisories can be found at: https://deb.freexian.com/extended-lts/