Add config/redis.yml unified config file

This MR adds a new config file config/redis.yml which is meant to eventually replace config/resque.yml, config/redis.shared_state.yml, config/redis.cache.yml, config/redis.queues.yml, etc.

Nothing changes in how we resolve Redis configuration unless the administrator opts in by creating a config/redis.yml file and putting configuration data in it. This is specifically designed to allow us to have a mixed deployment where some Redises use individual config files and some use redis.yml.

Part of gitlab-com/gl-infra/scalability#2063.