add support for running with private registry

Currently e2e tests don't support running on a cluster using a private registry.

To deploy an application on Workflow while using a private registry, the `PORT` env var needs to be set in the application config before a deploy can occur. E2E doesn't yet attempt to do this, so all such tests will fail with:

```
...
$ deis apps:create test-661286376 --no-remote
Creating Application... done, created test-661286376
If you want to add a git remote for this app later, use `deis git:remote -a test-661286376`
$ deis builds:create --app=test-661286376 deis/example-dockerfile-http
Creating build... Error: Unknown Error (400): {"detail":"PORT needs to be set in the application config when using a private registry"}
$ deis apps:destroy --app=test-661286376 --confirm=test-661286376
Destroying test-661286376...
done in 2s
...
```

Filing this issue so it is more readily known and perhaps to actually add support in future!