ceph-volume: use safer check for bluestore label

ceph-volume: use safer check for bluestore label

Using only the exit status of `ceph-bluestore-tool show-label` to
determine if a device is a bluestore OSD could report a false negative
if there is a system error when `ceph-bluestore-tool` opens the device.

A better check is to open the device and read the bluestore device
label (the first 22 bytes of the device) to look for the bluestore
device signature ("bluestore block device"). If ceph-volume fails to
open the device due to a system error, it is safest to assume the device
is BlueStore so that an existing OSD isn't overwritten.

Signed-off-by: Blaine Gardner <blaine.gardner@redhat.com>