Support Google Cloud Memorystore by disabling Redis CLIENT

Google Memorystore is a Redis instance that has disabled commands such as CLIENT and MONITOR. Sidekiq calls CLIENT to set an ID for debugging, but this can be disabled by setting the id to nil (https://github.com/mperham/sidekiq/issues/3521).

This is done in /etc/gitlab/gitlab.rb via:

gitlab_rails['redis_enable_client'] = false

By default, this is set to true.

Closes #4149 (closed)