* Add RabbitMQ alerts

Add Prometheus rules for RabbitMQ and cluster operator.
Add Alertmanager config for Slack notifications.
Add RabbitMQ-Alerts Grafana dashboard.
Co-authored-by: Gerhard Lazu <gerhard@lazu.co.uk>
Co-authored-by: Michal Kuratczyk <mkuratczyk@pivotal.io>

* Fix typo

* Clarify that some labels are provided by the PromQL expression

* Don't use match label selectors in Prometheus spec

* Add output how to open UIs by port-forwarding
Co-authored-by: Gerhard Lazu <gerhard@lazu.co.uk>
Co-authored-by: Michal Kuratczyk <mkuratczyk@pivotal.io>