Simply use class names as option help

The justification for this is that 1) the options are
already documented in the API documentation and 2) the
class names are fairly self-explanatory, so adding short
descriptions doesn't add much.

In the future, however, we could add a short description
by simply using the first line or sentence of the docstring;
note that docstrings would have to be canonicalized, which
they aren't now.

mininet/node.py

@@ -906,7 +906,7 @@ class Switch( Node ):
            deleteIntfs: delete interfaces? (True)"""
         if deleteIntfs:
             self.deleteIntfs()
-    
+
     def __repr__( self ):
         "More informative string representation"
         intfs = ( ','.join( [ '%s:%s' % ( i.name, i.IP() )
@@ -1517,3 +1517,7 @@ def DefaultController( name, controllers=DefaultControllers, **kwargs ):
     if not controller:
         raise Exception( 'Could not find a default OpenFlow controller' )
     return controller( name, **kwargs )
+
+def NullController( *_args, **_kwargs ):
+    "Nonexistent controller - simply returns None"
+    return None

mininet/topo.py

@@ -318,6 +318,12 @@ class SingleSwitchReversedTopo( Topo ):
                           port1=0, port2=( k - h + 1 ) )
 
 
+class MinimalTopo( SingleSwitchTopo ):
+    "Minimal topology with two hosts and one switch"
+    def build( self ):
+        return SingleSwitchTopo.build( self, k=2 )
+
+
 class LinearTopo( Topo ):
     "Linear topology of k switches, with n hosts per switch."