Disable NIC offloading such that this NAT can be used with user space OVS

Change-Id: I991c069e647107fd8de8f7e474da209ea215aff1
diff --git a/trellis/trellis_hybrid_v4.py b/trellis/trellis_hybrid_v4.py
index 2db3b77..ae2470c 100755
--- a/trellis/trellis_hybrid_v4.py
+++ b/trellis/trellis_hybrid_v4.py
@@ -10,10 +10,9 @@
 from mininet.cli import CLI
 from mininet.log import setLogLevel
 from mininet.node import OVSBridge, OVSSwitch
-from mininet.nodelib import NAT
 from ipaddress import ip_network
 from routinglib import BgpRouter
-from routinglib import RoutedHost
+from routinglib import RoutedHost, UserNAT
 from trellislib import DhcpClient, DhcpServer
 from trellislib import DualHomedDhcpClient
 from trellislib import get_mininet, set_up_zebra_config
@@ -109,7 +108,7 @@
         cs0 = self.addSwitch('cs0', cls=OVSBridge, datapath='user')
 
         # Control plane NAT (for quagga fpm)
-        nat = self.addHost('nat', cls=NAT,
+        nat = self.addHost('nat', cls=UserNAT,
                            ip='172.16.0.1/24',
                            subnet=str(ip_network(u'172.16.0.0/24')), inNamespace=False)
         self.addLink(cs0, nat)