Support private Hydra projects

[Ma27]

Description

WIP on purpose as it doesn't make sense to merge this without https://github.com/NixOS/hydra/pull/919

Also, before we merge this, it needs some final polishing (see also the unchecked items below).

Checklist

• [x] Built with make build
• [ ] Formatted with make fmt (goimports doesn't seem to exist when I run nix-shell even though nixpkgs is pinned)
• [ ] Verifed the example configuration still parses with terraform init && terraform validate (you may need to make install from the root of the project) (not yet, but I verified it with a dump of my Hydra settings that I applied against localhost:63333)
• [ ] Ran acceptance tests with HYDRA_HOST=http://0.0.0.0:63333 HYDRA_USERNAME=alice HYDRA_PASSWORD=foobar make testacc (you will need to spin up a local / temporary instance of Hydra)
• [ ] Added or updated relevant documentation (leave unchecked if not applicable)

[grahamc]

Overall it looks pretty good to me. Before we merge we should make sure @cole-h gets a review in as well.

[Ma27]

@grahamc please keep in mind that the Hydra PR needs to be merged first :)