ceph-disk: fix for journal symlink missing when re-using journal partition

We observe that the new /dev/disk journal symlink is not always
created when reusing a journal partition. To workaround, we check
if the symlink exists, trigger udev if needed, then assert that
the symlink exists (since progressing further in ceph-disk will be
useless in this case).

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>