os/bluestore/ceph-bluestore-tool: Modify show-label for many devs

os/bluestore/ceph-bluestore-tool: Modify show-label for many devs

It was possible to give multiple devices to cbt:
> ceph-bluestore-tool show-label --dev /dev/sda --dev /dev/sdb

But is any of devices cannot provide valid label, nothing was printed.

Now, always print results. Non readable labels are output as empty dictionaries.
Exit code:
- 0 if any label properly read
- 1 if all labels failed

Fixes: https://tracker.ceph.com/issues/68505
Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>