Option to prevent dd on compute by deleting lv

To increase the performance of any instances I've tested in combination with libvirt and kvm some cases. KVM + Block devices seems to be the fastest but I've found a show stopper. Every time if i delete an instance nova will do a dd on the deleted volume and this takes a long time and consume a lot of power.

I've found in the code (https://github.com/openstack/nova/blob/master/nova/virt/libvirt/utils.py#L350) a comment to make this configurable as by cinder volumes (scure delete). This should be done otherwise is lvm on a compute in a production system in my cases not usable.

Cheers
Heiko