certificates: generate libvirt TLS certificates

Adds support to the 'kolla-ansible certificates' command for generating
certificates for libvirt TLS, when libvirt_tls is true. The same
certificate and key are used for the libvirt client and server.

The certificates use the same root CA as the other generated
certificates, and are written to
{{ node_custom_config }}/nova/nova-libvirt/, ready to be picked up by
nova-libvirt and nova-compute.

Change-Id: I1bde9fa018f66037aec82dc74c61ad1f477a7c12