Refactors GreenBUildRepository to use named parameters
$ bookbinder push_to_prod namespaces uploaded tarballs [Finishes #63656880]

- Raise friendly errors where required config values or ENV variables might be missing.
- Stub Pusher#push in spec_helper to prevent shell-out network requests that webmock won't catch!
Refactors DocRepo#name into Repository#short_name