basic ovs support

these are backward compatible changes (json config and usage of the tool) where 'bridge' means default linux bridge, where as a new option for network 'ovs-bridge' is actually a bridge in the base class, but is handled differently when creating a bridge and adding ports to it.

The other design choice was to create ovs-bridge a different class, but except a few lines of differences both linux bridge and ovs-bridge share the same code.