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 fsid file, it will however
override it.

Instead of overriding the files in the osd data directory, return
immediately if the magic file exists.

Signed-off-by: Loic Dachary <loic@dachary.org>