Add OS::Contrail::VirtualNetwork

Change-Id: I627a77016eed43cc9698a807a035b74d5cd67722
Please see https://github.com/Juniper/contrail-controller/blob/master/src/config/vnc_openstack/vnc_openstack/neutron_plugin_db.py +1099
method _network_neutron_to_vnc which is used by method network_create.
The VirtualNetwork object is created without forwarding-mode (it defaults to neutron).
In general looking at the neutron code above is a good way to figure out how to generate the heat resources. We only use forwarding-mode in some very specific use cases.
The default forwarding-mode is Neutron compatible; we have a non-Neutron forwarding mode (L2only) that is used for very limited use cases.