Fix #2051: preserve DB/WAL disk on destroy

Fix #2051: preserve DB/WAL disk on destroy

When destroying an OSD over API or CLI, e.g. by executing:

'pveceph osd destroy <num> --cleanup'

all disks associated with the OSD got wiped with dd, which included
any shared and by others still in use ones, e.g., separate disks with
DB/WAL.

The patch changes 'wipe_disks' to wipe the partition instead of the
whole disk.

Signed-off-by: Alwin Antreich <a.antreich@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>