fix(controller): Determine registry address during start

Before this change re-deploying apps failed when the registry was re-scheduled to a different instance in the cluster.

Fixes #1128
At which point is the controller interacting with docker? The only place I found where this image reference is used is here: https://github.com/deis/deis/pull/1325/files#diff-530a37eb84d4183cb52cef27ecce6694R224 That's why I used a simple fixed name and replace with the actual ip instead of doing more fancy dns things to resolve the current registry node within coreos.