Sidekiq cluster service should be independent from sidekiq service

We ran into issues with https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1660 when we tried to run sidekiq-cluster without a sidekiq service. This is a bit unexpected and expected at the same time. Expected because sidekiq-cluster was meant to be a wrapper around sidekiq. Unexpected because we should be able to run it independently from the general sidekiq service.