Before this change, Rails cache data was stored in a global Redis
namespace. As a consequence, clearing the Rails cache (`rake
cache:clear`) would also delete all Sidekiq queue data and session
storage. This change puts all Rails cache data in a `cache:gitlab`
namespace, making `rake cache:clear` safe again.
It was not possible to start Gitlab with a redis server running on another host or port. Every other subsystem that uses redis respects the settings in config/resque.yml.
This patch uses the configured url from the cache_store configuration.
The cache_store configuration already loads the correct redis url from config/resque.yml.
Closes#5406