* Integrate RabbitMQ Alerting without Prometheus Operator

Before this commit, RabbitMQ Alerting was set up with the assumption
that Prometheus Operator is installed.
However, many K8s clusters have Prometheus installed without using
Prometheus Operator.
Therefore, support RabbitMQ alerting integration with and without using Prometheus Operator.

Auto-generate single Prometheus rule file from PrometheusRules.
Add Prometheus RabbitMQ scrape targets (do not rely on outdated
annotation approach).

* Remove line to set dev image since v1.7.0 will be cut very soon

* Make clear that Prometheus CRDs can be put in any namespace

...NamespaceSelector {} means any namespace will be selected.

This is not to be confused with value `nil` which means "only check own
namespace".

* Skip SSL verification in dev setup

The bug got introduced in https://github.com/rabbitmq/cluster-operator/pull/671

* Update app.kubernetes.io/component labels on resources

* Correct component label on service monitor example

* 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>