Fix issue with old ceph-disk-prepare

Old ceph-disk-prepare re-write the partition table and fail before writting
the Parition GUID.
This fix tests the presence of a osd_device status, if present, we skip
the OSD.
It also fix a typo when setting the osd_device status

Signed-off-by: Alexandre Marangone <alexandre.marangone@inktank.com>
Merge pull request #18 from alram/master

Fix for old ceph-disk-prepare