ceph-disk: set guid if reusing a journal partition

When reusing a journal partition (e.g. /dev/sda2) we should set a
new partition guid and link it correctly with the OSD. This way
the journal is symlinked by its persistent name and ceph-disk list
works correctly.

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
Maybe factorize the two to highlight the differences ? --new + journal if not reusing vs no --new and base if reusing