ceph-disk: don't change the journal partition uuid

We observe that the new /dev/disk/by-partuuid/<journal_uuid>
symlink is not always created by udev when reusing a journal
partition. Fix by not changing the uuid of a journal partition
in this case -- instead we can reuse the existing uuid (and
journal_symlink) instead. We also now assert that the symlink
exists before further preparing the OSD.

Fixes: #10146
Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
Tested-by: Dan van der Ster <daniel.vanderster@cern.ch>