Add `redis_instance` entry to /etc/hosts for easy access

By adding redis_instance to the /etc/hosts file, application configuration can access the alias rather than needing to alter configs of every process that needs access to redis.  This change is designed to work for solo, cluster, app instance access, util instance access (eg. resque in our case) and is guarded for environments that do not even have a redis instance.

I also cleaned up some outdated references in the docs.