Conditionally start sidekiq-cluster named Sidekiq

When configuring Sidekiq to run in cluster mode, using sidekiq['cluster'] = true, we would previously start up a service called sidekiq-cluster. This might be confusing to administrator, as the service they configured is sidekiq rather than sidekiq-cluster.

This changes that behaviour, if sidekiq-cluster was started using the new configuration:

  sidekiq['enable'] = true
  sidekiq['cluster'] = true

Then the service that will run sidekiq-cluster will be called sidekiq rather than sidekiq-cluster

Configuring sidekiq-cluster using the soon-to-be deprecated configuration

  sidekiq_cluster['enable'] = true

would still result in a service called sidekiq-cluster.

It is not possible to start sidekiq-cluster twice with conflicting configuration. So we know when node['gitlab']['sidekiq']['cluster'] is set and sidekiq_cluster is enabled that this will be the only sidekiq-cluster process that will be started.

Demo of the configuration on a test instance: https://stuff.vanlanduyt.co/file/dropstuff/cluster-process-naming-gQNcCz2Ene5Vw9wQZcZrF9rFVNWU6G.mov

gitlab-com/gl-infra/scalability#249