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.
Merge pull request #1058 from deis/scale-before-push

fix(controller): set default release image