Switch to RabbitMQ management docker image variant

[Zerpet]

Is your feature request related to a problem?
The non-management image variant disabled management agent metric collection. This configuration is set in conf.d and it takes precedence over any configuration set in rabbitmq.conf. This means that we can't override this configuration using the RabbitmqCluster spec.

Describe the solution you'd like
The image variant 3.8-management comes with the management plugin enabled and it does not disable the metric collection. Given that we want to always provide the management UI, it makes sense that we use this image instead. We should set this image by default in the Operator.

Describe alternatives you've considered
N/A

Additional context
Discussed in weekly sync up ✌️