Servers model interface

This is first set of PRs based on discussion we all had w.r.t servers table spike in #678. Please review this.

Please ignore the PR build failed that will be fixed after merging #683 
Are we keeping this totally internal for now? Since the whole id vs nova_id and "status" thing is going away with the converger, it might be a good idea to keep the number of things that count on it working that way to a minimum.