Use ip route to determine default intf for port forwarding

We were using eth0 by default before, but this doesn't work if
your default interface isn't eth0.

Fixes port forwarding on Ubuntu 16.04.

Change-Id: I55baed7fd8952a9f6cab364e20e31a0632ddda6d

@@ -55,6 +55,8 @@ from sys import argv
 from glob import glob
 import time
 from functools import partial
+from re import search
+
 
 ### ONOS Environment
 
@@ -418,9 +420,22 @@ class ONOSCluster( Controller ):
         "Return list of ONOS nodes"
         return [ h for h in self.net.hosts if isinstance( h, ONOSNode ) ]
 
-    def configPortForwarding( self, ports=[], intf='eth0', action='A' ):
-        """Start or stop ports on intf to all nodes
+    def defaultIntf( self ):
+        "Call ip route to determine default interface"
+        result = quietRun( 'ip route | grep default', shell=True ).strip()
+        match = search( r'dev\s+([^\s]+)', result )
+        if match:
+            intf = match.group( 1 )
+        else:
+            warn( "Can't find default network interface - using eth0\n" )
+            intf = 'eth0'
+        return intf
+
+    def configPortForwarding( self, ports=[], intf='', action='A' ):
+        """Start or stop forwarding on intf to all nodes
            action: A=add/start, D=delete/stop (default: A)"""
+        if not intf:
+            intf = self.defaultIntf()
         for port in ports:
             for index, node in enumerate( self.nodes() ):
                 ip, inport = node.IP(), port + index