Using external Ceph with Kolla means there is no storage node for
kolla, but cinder-volume must be run on nodes(it can be controller
nodes or compute nodes), and the external-ceph-guide miss storage node,
this patch to fix it.
Closes-Bug: #1740028
Change-Id: I6cfb86c5c0d7b30291bd9f263fb030e4703d9602