let reactive routing support IPv4 default route 0.0.0.0/0

Before move reactive routing to new config-subsystem, this function does not work. Since there is no flow for host inside SDN network to go to the default peer router(which announced 0.0.0.0)

But I just setup the environment test again, it just works. Why it just works? because when move to the new config system, I changed a function:
SDN-IP treats all the interfaces equal, did not diff which are external interfaces to peers, which are internal to hosts inside SDN network.

SDN-IP app used only install flows for interfaces connected to peer with sdn-ip.json and addresses.json. The rest internal interfaces which are configured with IP prefixes, SDN-IP app does not care. Since right now, when we config the internal interfaces inside network-cfg.json, SDN-IP will treat them as equal. So when SDN-IP receives 0.0.0.0, it also installs flows for the internal interfaces.

So finally, I do not need to right new code to let it work.