Using `exit 0` in the ntpd stub causes some unwanted log warnings:
maas.service_monitor[151]: [warn] Service 'ntp' is on but not in the
expected state of 'running', its current state is 'exited'.
This change allows the stub to respond appropriately to 'systemctl
status ntpd' and 'systemctl restart ntpd' and keeps MAAS happier.
Change-Id: I41b95051ce595fb9001f4104a1abb48b66a657c4