don't break if node["ceph"]["osd_devices"] is empty (but log it)
remove useless create file from chef value, read variable, create file, remove file...
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Merge pull request #23 from Youscribe/error_osd_device

don't break if node["ceph"]["osd_devices"] is empty

Reviewed-by: Alexandre Marangone <alexandre.marangone@inktank.com>