gui_topology: fix to work using WebSocket

Hello All.

I midified ryu.topology.js to dinamicaly add or remove switch
using WebSocket on Ryu Topology Viewer.
Sorry, I had sent a pull request but it was closed.

Signed-off-by: Yoshiharu Yamashita <dyson.yamashita@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>