Monkeypatch and reopen eventlet's hub in novnc

eventlet.monkeypatch was not done in nova-novncproxy, but we need it in
order to do an rpc call and validate the token.

Also,With this we reopen eventlet's hub after a fork (triggered from
websockify when a new client connects) to prevent sharing epoll's fd
with the parent, which may cause erratic behaviour.

This caused novncproxy to stop working when it had more than two
clients connected.

Fixes bug #1193031

Change-Id: Ib25cb13273de807f9719bc1403684e81f5cb125e