refactor(controller): optimize layer creation

This restructures the Dockerfile in such a way that allows users to rebuild the
controller with minimal changes to the docker image's layers.
The idea with the redundant `pip install` command was that we might cache that layer, so that the later `pip install -r requirements.txt` would be close to a no-op. It's after a cache-busting `ADD` instruction, so now users rebuilding deis/controller will download every python package on every rebuild. I'm not sure that's what we want.