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

network/service_net_map.j2.yaml

@@ -64,6 +64,8 @@ resources:
                 {{network.name_lower}}: {get_param: {{network.name}}NetName}
 {%- endfor %}
               keys: {get_param: ServiceNetMapDeprecatedMapping}
+        # (mschuppert) this is to keep NovaVncProxyNetwork and NovaLibvirtNetwork
+        # in sync to not break VNC console
         - NovaVncProxyNetwork: { get_param: [ServiceNetMap, NovaLibvirtNetwork] }
 
 outputs: