Add OS::Contrail::VirtualNetwork

Change-Id: I627a77016eed43cc9698a807a035b74d5cd67722
web-ui doesn't allow to use "None" forwarding-mode and there is no default value in contrail-controller itself AFAIK - https://github.com/Juniper/contrail-controller/blob/master/src/schema/vnc_cfg.xsd#L659 and L643 - I can't check it now but will network work properly with it unset ?