restructured defaultController into a function

bin/mn

@@ -25,7 +25,7 @@ from mininet.cli import CLI
 from mininet.log import lg, LEVELS, info, debug, error
 from mininet.net import Mininet, MininetWithControlNet, VERSION
 from mininet.node import ( Host, CPULimitedHost, Controller, OVSController,
-                           NOX, RemoteController, DefaultController, UserSwitch, OVSKernelSwitch,
+                           NOX, DefaultController, RemoteController, UserSwitch, OVSKernelSwitch,
                            OVSLegacyKernelSwitch, IVSSwitch )
 from mininet.link import Link, TCLink
 from mininet.topo import SingleSwitchTopo, LinearTopo, SingleSwitchReversedTopo