Title: ntp.time: better handling of temporary synchronization loss
Level: 2
Component: checks
Class: fix
Compatible: compat
State: unknown
Version: 1.2.8b4
Date: 1456760686

After a reboot of the target machine or of the NTP server the check always
go CRIT for a couple of minutes - since NTP disregarded the peers as suitable.
Now this is being tolerated for a configurable amount of time. Per default
after 5 minutes of being without a valid time source the check goes WARN
and after 60 minute CRIT - but only if before this there was a valid time.
