Testable routing 188

Fixes #188 
Maybe move `Proxy` to a `flocker.route._model` module as you've done for the *record* types `flocker.node` in https://github.com/ClusterHQ/flocker/compare/flocker-cli-deploy-19#diff-5 and now in https://github.com/ClusterHQ/flocker/blob/master/flocker/node/_model.py