Set memory override value in RabbitMQ configuration

[Zerpet]

Is your feature request related to a problem?

The community RabbitMQ image in DockerHub sets the total memory available to the node in the entry point script, matching the memory defined by the cgroups. We want to stop relying on any behaviour that is implemented via this entry point script and make any relevant configuration part of the Operator configuration. This PR from rabbitmq-server delivered the ability to set the property total_memory_available_override_value to configure how much memory is available to RabbitMQ. By default, RabbitMQ will assume all the memory available in the current host. This becomes a problem in the context of containers because RabbitMQ will, incorrectly, assume that it has all the memory available in the host, however the memory will be limited by either cgroups or Kubernetes resource limits.

Describe the solution you'd like

Automatically configure the value of total_memory_available_override_value based on the value in .spec.resources.limits.memory. If this property is explicitly set to empty object i.e. unlimited, we will skip setting it and let the container assume it has all the memory available in the kubernetes node.