Respect the configured redis URL when initializing the session store

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
Merge pull request #5474 from vollnhals/patch-1

Respect the configured redis URL when initializing the session store