fix(controller): use build image for publishing releases

A release == build + config, not release + config. This change forces the builder to explicitly push a build tag, then tells the controller to use the last known build image, rather than tagging it as a release and bumping config on top of `latest`.

fixes https://github.com/deis/deis/issues/1818
That goes through the AppBuildViewSet, not the BuildHookViewSet. This view is only for use between the builder and the controller, IIRC. `Release.new()` handles `deis pull` operations via https://github.com/deis/deis/blob/master/controller/api/models.py#L634-L652