Libvirt debug logs can grow very large, particularly on Ubuntu (we have
seen up to 1.1GB). Disable libvirt debug logging using variable
nova_libvirt_logging_debug which was added to Kolla Ansible during the
Wallaby cycle (see change I90bfd1b300ad3202ea4d139fda6d6beb44c5820f) and
backported to Victoria and Ussuri.
Change-Id: Ic047e80aa382148b009b1c03581b417d17ec2598