add ceph-csi-rbd support for baremetal nodes

With Ref: https://gitlab.cern.ch/kubernetes/project/-/issues/955 we need to add ceph-csi-rbd charts to cern-magnum, define the required cluster user config and document the changes.
Here are the sample manifests: https://gitlab.cern.ch/kubernetes/project/-/issues/955#note_10355203