Force reopening eventlet's hub after fork

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: I3ff9001543b84b1037597da243422490bb611657
(cherry picked from commit cb25bc4530323aaa33d5c42eb01f998d463f2106)