Changes to group tests to get them to work on OVS.

Use barriers instead of sleeps, test multicast forwarding out 2 ONU ports, etc.
diff --git a/olt-topo.py b/olt-topo.py
index 438a59f..581084c 100755
--- a/olt-topo.py
+++ b/olt-topo.py
@@ -23,7 +23,7 @@
 
 if __name__ == '__main__':
     setLogLevel('debug')
-    topo = OltTopo()
+    topo = OltTopo(k=2)
 
     net = Mininet( topo=topo, controller=RemoteController )