ceph-disk: prepare --data-dir must not override files

ceph-disk does nothing when given a device that is already prepared. If
given a directory that already contains a successfully prepared OSD, it
will however override it.

Instead of overriding the files in the osd data directory, return
immediately if the magic file exists. Make it so the magic file is
created last to accurately reflect the success of the OSD preparation.

Signed-off-by: Loic Dachary <loic@dachary.org>