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
Merge "Force reopening eventlet's hub after fork"