Adjust CINDER_SECURE_DELETE flag to cinder changes

Cater for the pending more general cinder support
for configurable volume wiping method at:
https://review.openstack.org/#/c/12521

This change is done here first so as to not
trigger a CI lockup when the referenced patch above lands.
When that's in place, we can remove the older
secure_delete config adjustment in a subsequent patch.

Change-Id: I73fe2e0d1cf2815ab6025121584951cb5ff56fa3
Merge "Adjust CINDER_SECURE_DELETE flag to cinder changes"