Improve printer installation on teacherlaptops

- Update printer list when connected via VPN
- Prevent printer removal, when logging in without a connection to the school network

roles/lmn_printer/templates/install-printers.sh.j2

@@ -2,6 +2,9 @@
 
 set -eu
 
+## Exit if first printserver is not reachable
+ping -c1 -W1 {{ printservers | first }} || exit 0
+
 printservers="{{ printservers | join(' ') }}"
 hostgroup="$(id -Gn "${HOSTNAME^^}$")"
 usergroup="$(id -Gn "${SUDO_USER}")"