rbd: refuse to create block volumes
rbd: refuse to create block volumes

Without this check, the driver fails one of the E2E storage tests in
Kubernetes 1.13: provisioning a block volume is expected to fail in
https://github.com/kubernetes/kubernetes/blob/e689d515f77cda51898045d626ec4070e3328194/test/e2e/storage/testsuites/volumemode.go#L329-L330

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi