Allow setting sizeLimit when persistence disabled #2017

Is your feature request related to a problem? Please describe.

When no persistence is used with a RabbitmqCluster, the Operator sets an emptyDir volume for the persistence:

    - name: persistence
      emptyDir: {}

This causes that in the RabbitMQ UI, the available disk is the host's disk:

This introduces a potential problem in the host, since its disk may fill up and a RabbitMQ cluster can affect the rest workloads of the host.

Describe the solution you'd like

Kubernetes lets to set the sizeLimit for an empty dir: https://kubernetes.io/docs/concepts/storage/volumes/#emptydir-configuration-example

The Operator could add a sizeLimit when no persistence is used. Questions we ask ourselves:

• What is the default value for sizeLimit?
• Should the user modify this value? If so, shoud the CRD be modified to add a new field?
• What about other related fields, such as the disk threshold alarm?

Describe alternatives you've considered

• Use a default value for sizeLimit (500Mb) that can be overridden by an annotation.
• Have a disk usage limit alarm. RabbitMQ now defines the disk_free_limit.absolute alarm, which allows to define an alarm when the disk has less that the specified amount of space. We propose the opposite: setting an alarm when disk usage exceeds a specified amount of space.