Stub out RemoteOVSSwitch.batchShutdown()

Eventually we should implement true batch shutdown.
In the mean time, we just ignore it. Note there's no good
way that I know of for a subclass to remove a superclass
method, so we changed the protocol a bit to require a return
value of True.

mininet/net.py

@@ -505,8 +505,8 @@ class Mininet( object ):
         for swclass, switches in groupby(
                 sorted( self.switches, key=type ), type ):
             switches = tuple( switches )
-            if hasattr( swclass, 'batchShutdown' ):
-                swclass.batchShutdown( switches )
+            if ( hasattr( swclass, 'batchShutdown' ) and
+                 swclass.batchShutdown( switches ) ):
                 stopped.update( { s: s for s in switches } )
         for switch in self.switches:
             info( switch.name + ' ' )

mininet/node.py

@@ -1103,6 +1103,7 @@ class OVSSwitch( Switch ):
         quietRun( 'ovs-vsctl ' +
                   ' -- '.join( '--if-exists del-br %s' % s
                                for s in switches ) )
+        return True
 
     def dpctl( self, *args ):
         "Run ovs-ofctl command"