Remove support for configuring sidekiq_cluster next to sidekiq from omnibus

For after #239 (closed) we need to remove support for configuring sidekiq_cluster next to sidekiq. Configuring sidekiq cluster will need to be done through sidekiq['*'] in gitlab.rb.

1. sidekiq_cluster[*] is no longer a thing.
2. We stop using bundle exec sidekiq altogether.
3. Everything related to sidekiq is set using sidekiq[*]. sidekiq['enable'] decides if there is a sidekiq service or not - we don't even need sidekiq['cluster'].
4. The service spawned is sidekiq, but it will run sidekiq-cluster under the hood. It is managed by gitlab-ctl commands to sidekiq
5. The logs go to /var/log/gitlab/sidekiq.