Temporarily disable Cinder secure delete for snaps

Workaround for LVM on CentOS 6.4.
See:
    https://bugzilla.redhat.com/show_bug.cgi?id=975052
    https://bugs.launchpad.net/cinder/+bug/1191812
    https://review.openstack.org/#/c/45114
    https://review.openstack.org/#/c/45117