feat(*): allow custom component images

Pull image names for Deis components from etcd, defaulting to the
stock Deis component images. This enables users to supply their own
router, for instance, simply by setting an etcd key:

/deis/{component}/image

Also, the Deis release version is now stored in /etc/deis-release.
This is by default appended when getting the image names for Deis
components, but can be overridden if set in etcd at /deis/release.
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.
fix(controller): update Dockerfile maintainer
Merge pull request #1246 from deis/dockerfile-optimization

refactor(controller): optimize layer creation
Merge pull request #1123 from deis/1023-parameterized-units

feat(*): allow custom component images