Merge pull request #736 from hwchiu/FIX_TBF_LINK

Modify the default latency type from int to float if tbf is enabled.
Fixes #729

mininet/link.py

@@ -252,7 +252,7 @@ class TCIntf( Intf ):
                           + 'rate %fMbit ul rate %fMbit' % ( bw, bw ) ]
             elif use_tbf:
                 if latency_ms is None:
-                    latency_ms = 15 * 8 / bw
+                    latency_ms = 15.0 * 8 / bw
                 cmds += [ '%s qdisc add dev %s root handle 5: tbf ' +
                           'rate %fMbit burst 15000 latency %fms' %
                           ( bw, latency_ms ) ]