ceph-mon: change command to see if rbd exists
ceph-mon: change command to see if rbd exists

The previous command was hanging, see this issue:

https://github.com/ceph/ceph-ansible/issues/1440

Signed-off-by: Andrew Schoen <1bb641dc23c3a93cce4eee683bcf4b2bea7903a3@redhat.com>

fgal/ceph-ansible,jtaleric/ceph-ansible,travmi/ceph-ansible,bengland2/ceph-ansible,albertomurillo/ceph-ansible,ceph/ceph-ansible,travmi/ceph-ansible,ceph/ceph-ansible,font/ceph-ansible,fgal/ceph-ansible,font/ceph-ansible,jtaleric/ceph-ansible,albertomurillo/ceph-ansible,bengland2/ceph-ansible,albertomurillo/ceph-ansible