ceph-disk: dmcrypt file permissions

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>
Merge pull request #3087 from dachary/wip-9785-dmcrypt-keys-permissions

ceph-disk: dmcrypt file permissions

Reviewed-by: Sage Weil <sage@redhat.com>