Add EE sidekiq_cluster configurable for setting up extra Sidekiq processes
This allows configuring the settings that were added in: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/922
cc\ @yorickpeterse
This allows configuring the settings that were added in: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/922
cc\ @yorickpeterse