Improve NTP handling:
- Drop specific NTP configuration on subnets behind the new firewall
- Add the public debian ntp pool to the sesi_rocquencourt subnet
- Add an icinga check for ntp synchronization on all linux hosts
Related to T3063
Test Plan
tested NTP changes on worker01, icinga changes on pergamon
Migrated from D5126 (view on Phabricator)