NTP functionnal tests: move + note

- Move the NTP tests to spof class, since NTP is required to build the
  Pacemaker cluster.
- Add a note which says that the package is not yet in Debian:
  https://ftp-master.debian.org/new/ntpstat_0.0.0.1-1.html

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>