refactor to be deployed as an app on workflow

Right now if you attempt to add the following envvars and disable routing, the application won't deploy because it relies on the hard-coded secret paths:

```
><> deis create postgres
Creating Application... done, created postgres
Git remote deis successfully created for app postgres.
><> deis routing
Routing is disabled.
><> deis config:push
Creating config... done

=== postgres Config
AWS_ACCESS_KEY_ID ****************
AWS_DEFAULT_REGION us-west-1
AWS_SECRET_ACCESS_KEY *************
BUCKET_NAME test-bucket
DATABASE_STORAGE s3
PORT 5432
><> deis pull quay.io/deis/postgres:v2.2.1
Creating build... ...^C
```

I'd expect that with the above environment variables, deis/postgres should be deployable on Workflow as a standalone app.