Check_MK: time sync check

Checking fluctuation of system time difference between servers can be done by Check_MK's local checks. Start NTP service on primary server, then `ntpdate -q` the primary server to get the offset interval. Set WARNING and CRITICAL values in your local check script, either shell or perl script, then all is done. Monitoring interval better set one hour or more.

REF: https://mathias-kettner.de/checkmk_localchecks.html