GitLab::Redis::{Cache,Queues,SharedState} have a configuration
fallback mechanism where in the absence of either YAML configuration
file, they fall back to localhost Redis URL's. These default URL's
make no sense because if you need multiple Redis instances, your
GitLab is too big to have Redis on localhost.

Because there is a remote chance someone is using these default URL's
anyway, this commit stops short of removing them. But we do refactor
the default URL lookup code to make it unlikely that the next person
adding a new Redis instance to the app will cargo-cult the default URL
behavior.