Set batch=False in OVSSwitch for low-level API

If you try to use the low-level API, you are probably
not going to call batchStartup()! So, we set batch=False
by default. This means that buildFromTopo() needs to set
it to True, so we add a bit of irritatingly complex machinery
to allow this to happen. The good fallout of this is that
now customConstructor() returns a real subclass, not simply
a constructor function! We also detect errors where people
are incorrectly attempting to give parameters to a lambda
function - since none of our lambdas accept parameters!!

Note that this is a bit like functools.partial for classes -
it would be nice if functools had a true subclassing function.

mininet/net.py

@@ -414,7 +414,12 @@ class Mininet( object ):
 
         info( '\n*** Adding switches:\n' )
         for switchName in topo.switches():
-            self.addSwitch( switchName, **topo.nodeInfo( switchName) )
+            # A bit ugly: add batch parameter if appropriate
+            params = topo.nodeInfo( switchName)
+            cls = params.get( 'cls', self.switch )
+            if hasattr( cls, 'batchStartup' ):
+                params.setdefault( 'batch', True )
+            self.addSwitch( switchName, **params )
             info( switchName + ' ' )
 
         info( '\n*** Adding links:\n' )