{Reviewer: Andy} Scaling down

Andy, this branch adds code to:
* Quiesce bono/sprout/cassandra nodes when scaling a deployment down
* Terminate those nodes when calling "--finish" if they have quiesced
* Unquiesce those nodes if the deployment gets resized back to its original size
* Prevent any other resize to a quiescing deployment
* Destroy stopped nodes with "knife deployment clean" (fixes https://github.com/Metaswitch/chef/issues/58)

Can you review?

Note two restrictions: I'm not doing anything with memcached, and I don't have any code to notice that e.g. SIPp nodes don't need quiescing and to just delete them immediately (though they are cleaned up on a --finish). I'm not convinced that special handling for SIPp nodes is worth it.