[r=wallyworld] New assignment policy is AssignCleanEmpty
Add container support to AssignClean(Empty) policies and make the default policy AssignCleanEmpty. This allows manual deployment of charms into existing clean and empty containers/instances, with the fallback that a new container/instance is created if none are available. The system behaves the same as previously if a simple bootstrap and deploy is performed, but if add-machine is used to create some containers/instances, then those are no longer ignored.