libvirt: support configurable wipe methods for LVM backed instances

Provide configurable methods to clear these volumes.
The new 'volume_clear' and 'volume_clear_size' options
are the same as currently supported by cinder.

* nova/virt/libvirt/imagebackend.py: Define the new options.
* nova/virt/libvirt/utils.py (clear_logical_volume): Support the
new options. Refactor the existing dd method out to
_zero_logic_volume().
* nova/tests/virt/libvirt/test_libvirt_utils.py: Add missing test cases
for the existing clear_logical_volume code, and for the new code
supporting the new clearing methods.
* etc/nova/nova.conf.sample: Add the 2 new config descriptions
to the [libvirt] section.

Change-Id: I5551197f9ec89ae2f9b051696bccdeb1af2c031f
Closes-Bug: #889299