lib/ovs/bridge: Return multiple controllers

OVSBridge may have more than one controller. Let get_controller()
return a list of controllers in such cases.

NOTE: this introduces incompatibility if multiple controllers are
configured for a bridge.

Signed-off-by: IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/lib/ovs/bridge.py

@@ -171,7 +171,8 @@ class OVSBridge(object):
         """
         command = ovs_vsctl.VSCtlCommand('get-controller', [self.br_name])
         self.run_command([command])
-        return command.result[0]
+        result = command.result
+        return result[0] if len(result) == 1 else result
 
     def set_controller(self, controllers):
         """