Don't create PVC if spec.persistence.storage==0

[mkuratczyk]

For test environments, especially in CI/CD, it may be beneficial to skip
creating the PVC altogether since the whole cluster is disposable
anyway. With this change, if spec.persistence.storage is set to 0, the
PVC is not created and an emptyDir volume is used instead. Changing from
0 to anything else is not supported (and doesn't make much sense
anyway).

This closes #776

[ChunyiLyu]

Everything works as expected. For an edge case, if someone set persistence.storage to 0 but defines PVCs through statefulSet override, they do not get any PVC created. Example manifest:

apiVersion: rabbitmq.com/v1beta1
kind: RabbitmqCluster
metadata:
  name: multiple-disks
spec:
  replicas: 1
  persistence:
    storage: 0
  override:
    statefulSet:
      spec:
        volumeClaimTemplates:
          - apiVersion: v1
            kind: PersistentVolumeClaim
            metadata:
              name: persistence
              namespace: default
            spec:
              accessModes:
                - ReadWriteOnce
              resources:
                requests:
                  storage: 10Gi
              volumeMode: Filesystem

Is this the desired behavior? If yes, may I suggest to add a unit level test for this behavior to capture in the code? We are getting this behavior because the code block of

if instance.Spec.Persistence.Storage.Cmp(zero) == 0 {
		return []corev1.PersistentVolumeClaim{}, nil
	}

is before override was applied and would be great to add a test to ensure so we don't change the behavior by accident.