Moved role constraints into model and removed edit deployment

(cherry picked from commit 089945280adff660e126252f74e5574e6431aee0)
Merge pull request #60 from vuntz/release/roxy/master

Moved role constraints into model and removed edit deployment
Removed the single assignment block from constraints
Merge pull request #61 from tboerger/feature/roxy/fix-constraints

Removed the single assignment block from constraints
basic HA support

needs reachable external NTP servers
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
on NTP servers always use local clock as fallback

to avoid trouble when external servers are unreachable.
Merge pull request #58 from SUSE-Cloud/ntpha

NTP High Availability