ceph-config: don't check for devices on existing osds

ceph-config: don't check for devices on existing osds

When osd_auto_discovery is true the `devices` var will be empty (as the disks have holders).
Also in general there is no need to check for devices to list the devices with ceph-volume as we have `default({})` on the stdout in `num_osds` set fact in the next task

Signed-off-by: Seena Fallah <seenafallah@gmail.com>