libvirt: add nova-libvirt-cleanup command

Change Ia1239069ccee39416b20959cbabad962c56693cf added support for
running a libvirt daemon on the host, rather than using the nova_libvirt
container. It did not cover migration of existing hosts from using a
container to using a host daemon.

This change adds a kolla-ansible nova-libvirt-cleanup command which may
be used to clean up the nova_libvirt container, volumes and related
items on hosts, once it has been disabled.

The playbook assumes that compute hosts have been emptied of VMs before
it runs. A future extension could support migration of existing VMs, but
this is currently out of scope.

Change-Id: I46854ed7eaf1d5b5e3ccd8531c963427848bdc99