Also do not try to rm the file if it does not exist (it seems to complain
and then exit if you try to do it on a non-existant file - which means
the function is not named nicely as -f to rm should make it ignore this)

This allows draksec to override things properly under polkit.

v2 (by Colin Guthrie)
  * Fix external variable reference (my -> our + $ when using it)
  * Drop systemd cgroup filesystem check as the path has changed now
    (prefer "systemctl is-active foo.service" instead)
  * Add iburst to chrony config server definitions (triggers sync at
    startup)
  * Don't use ntpdate with chrony - rather chronyc waitsync
  * Don't bother checking for ntp in finish-install - just use chrony