Test and document using the traefik docker image with an xwiki cluster

See https://traefik.io/

The ideas is to use XDOCKER-51 to setup an xwiki cluster and then using Traefik to load balance between the xwiki instances.

The goal of this issue is to test it and document how to put it in action. Should result in 5 docker commands:

• 1 - create the docker network
• 2 - start the DB
• 3 - start xwiki node 1
• 4 - start xwiki node 2
• 5 - start traefik