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

config/initializers/session_store.rb

@@ -2,6 +2,7 @@
 Gitlab::Application.config.session_store(
   :redis_store, # Using the cookie_store would enable session replay attacks.
+  servers: Gitlab::Application.config.cache_store.last, # re-use the Redis config from the Rails cache store
   key: '_gitlab_session',
   secure: Gitlab::Application.config.force_ssl,
   httponly: true,