Add comment for libvirt/vnc network sync
Add comment for libvirt/vnc network sync

In I08fa1ee947574e6fac24885b17706a22054fcba9 we added sync of
NovaLibvirtNetwork and NovaVncProxyNetwork. This just adds a
comment for this.

Change-Id: If8c74d5c00fb6b31de98917b984a376fc192ff53

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