Use None instead of localhost

See #211.  Some don't have localhost mapped, and
we should also bind to IPv6 interfaces.

Thanks to gaoyaxing24 for testing.

server/controller.py

@@ -198,7 +198,7 @@ class Controller(util.LoggedClass):
     async def main_loop(self):
         '''Controller main loop.'''
         if self.env.rpc_port is not None:
-            await self.start_server('RPC', 'localhost', self.env.rpc_port)
+            await self.start_server('RPC', None, self.env.rpc_port)
         self.ensure_future(self.bp.main_loop())
         self.ensure_future(self.wait_for_bp_catchup())