Geo: Make it possible to use internal URL with HTTPS

This is a spin-off of the discussion at https://gitlab.com/gitlab-org/gitlab-ee/issues/12144#note_187794816:

This way, we only need a load balancing with TLS terminating with we want to use an internal URL with HTTP(S).

I'm not sure how many customers would feel comfortable running HTTP internally without encryption. I suspect that's the exception, not the rule, especially if sensitive data is being transmitted. I think we'll want to support TLS internally as well.