ceph-volume: fix 'ceph_device' field in inventory

ceph-volume: fix 'ceph_device' field in inventory

`ceph-volume inventory` should display a boolean value for the field
'ceph_device'.

current result:

```
[root@6bda9ce84c4d /]# ceph-volume inventory /dev/vda

====== Device report /dev/vda ======

     path                      /dev/vda
     ceph device               None

```

expected result:

```
[root@6bda9ce84c4d /]# ceph-volume inventory /dev/sda

====== Device report /dev/sda ======

     path                      /dev/sda
     ceph device               True

[root@6bda9ce84c4d /]# ceph-volume inventory /dev/vda

====== Device report /dev/vda ======

     path                      /dev/vda
     ceph device               False

```

Fixes: https://tracker.ceph.com/issues/57100
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>