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
`target_image = "{{ .deis_registry_host }}:{{ .deis_registry_port }}/{app}"`. Pushing the app image's name to the controller requires the fully qualified path to the image, whereas here you're essentially specifying that the image lives on DockerHub at `deis-registry/{app}`, which isn't true.