Allow optional volumes for nova_libvirt container
Allow optional volumes for nova_libvirt container

Currently there in no heat parameter available to specify optional
volumes for nova_libvirt container.
This patch will allow to attach optional volumes to the nova_libvirt
container through 'NovaLibvirtOptVolumes' heat parameter.

Change-Id: I0651b93fe1e05ca674a3d349a114b980c2005eff
Closes-Bug: #1897265

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates