ceph-volume: fix error 'KeyError' with inventory

ceph-volume: fix error 'KeyError' with inventory

The tag ceph.cluster_name is always set at the end.
The only way it could be absent was if the osd prepare
has been interrupted between [1] and [2].

[1] https://github.com/ceph/ceph/blob/v14.2.11/src/ceph-volume/ceph_volume/devices/lvm/strategies/bluestore.py#L355-L387
[2] https://github.com/ceph/ceph/blob/v14.2.11/src/ceph-volume/ceph_volume/devices/lvm/prepare.py

Although the code received tremendous changes meantime
and this error shouldn't show up again, we need to handle
the case where this tag wouldn't have been set.

Fixes: https://tracker.ceph.com/issues/44356
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>