fix #1754: rbd: fix handling of external clusters

fix #1754: rbd: fix handling of external clusters

with the recent refactoring, external clusters were not handled
correctly with librd if a pveceph or storage specific ceph config
exists.

change the behaviour to include the pveceph config file only for pveceph
managed clusters, and a storage specific one only for external ones.

set mon_host correctly using the values from storage.cfg for external
librbd clusters.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>