loadbalancer: use 'source' for novnc balance mode

loadbalancer: use 'source' for novnc balance mode

When establishing a connection from the client (Web Browser) to the
novncproxy (loadbalanced by HAproxy), we need to make sure the client
will stick on the same server the time he's connected, because HAproxy
load-balance to another novncproxy node, the client will loose the
connection and timeout like 'Connection Reset By Peer error'.

This patch aims to configure novnc HAproxy configuration to balance
using 'source' mode, so it will make sure the server remains the same
while the connection is established.

Change-Id: Ibbb7162b763f1fd2854a10a92a681910e0683c0a
Closes-BZ: 1257324