refactor: let turbovnc listen to localhost

TigerVNC listens to localhost by default, and TurboVNC doesn't. Let's
make them do the same.

When websockify starts either TigerVNC or TurboVNC, it will listen only
to localhost anyhow - so this doesn't change anything really but could
reduce the complexity a bit when debugging these things, as I've done at
length now.

jupyter_remote_desktop_proxy/setup_websockify.py

@@ -38,7 +38,7 @@ def setup_websockify():
         vnc_args = [vncserver, '-rfbunixpath', sockets_path]
     else:
         websockify_args = []
-        vnc_args = [vncserver, '-rfbport', '{port}']
+        vnc_args = [vncserver, '-localhost', '-rfbport', '{port}']
 
     if not os.path.exists(os.path.expanduser('~/.vnc/xstartup')):
         vnc_args.extend(['-xstartup', os.path.join(HERE, 'share/xstartup')])