Merge pull request #62 from vuntz/release/stoney/master

Removed the single assignment block from constraints
basic HA support

needs reachable external NTP servers

(cherry picked from commit 6d714d8a0f62928c9038079f38deb7e7155626f0)
use IP addrs ; dont use own IP as server

using IP addrs for NTP avoids trouble from DNS being unavailable
and slightly reduces overall load

(cherry picked from commit 4ef02f10ffc4ebb57d51c7c300b35f28224cfa56)
on NTP servers always use local clock as fallback

to avoid trouble when external servers are unreachable.

(cherry picked from commit 15b2d5cc8e005abaf8212ec5ea909f3876c9b570)