Ensure sidekiq-cluster emits JSON logs

We can just re-use the Sidekiq JSON formatter to ensure this happens since sidekiq-cli doesn't use the Sidekiq initializer.

Part of #208922 (closed)

# cat /var/log/gitlab/sidekiq-cluster/current | grep Starting
{"severity":"INFO","time":"2020-03-04T14:13:26.463Z","message":"Starting cluster with 2 processes"}