ceph-disk: dmcrypt file permissions (giant)

The directory in which key files are stored for dmcrypt must be 700 and
the file 600.

http://tracker.ceph.com/issues/9785 Fixes: #9785

Signed-off-by: Loic Dachary <ldachary@redhat.com>
(cherry picked from commit 58682d1776ab1fd4daddd887d921ca9cc312bf50)