fix(controller): set default release image

If you scale an application with `deis scale cmd=1` before an
application has been pushed, it should deploy deis/helloworld.
However, the initial release (v1) does not have a image set for
the release, only the build. Setting the default to deis/helloworld
fixes the "scale before deploy" problem.

TESTING: rebuild the controller, then create an app and scale it:

```
$ deis create
$ deis scale cmd=1
```

You should see the helloworld app deployed to `<appname>.local.deisapp.com`.