Support subnet host-routes by adding InterfaceRouteTable to the ports

Nice algorithm !
I just remove some unused variables in '_port_update_prefix':
```python
def _port_update_prefixes(matched_route_lst, unmatched_host_routes):
    import ipdb; ipdb.set_trace()
    process_host_routes = True
    while process_host_routes:
        process_host_routes= False
        for route in unmatched_host_routes:
            ip_addr = route.get_next_hop()
            if IPAddress(ip_addr) in IPSet(matched_route_lst):
                matched_route_lst.append(route.get_prefix())
                unmatched_host_routes.remove(route)
                process_host_routes = True
```