zuul: let us change branch of zuul-config repo

The Zuul configuration is hosted in integration/zuul-config. Since the
production and labs environnements are entirely different, this patch
let us pass the branch of zuul-config we want to initially checkout.

The new parameter being config_git_branch (default: master). For labs
the branch is 'labs'.

Change-Id: Ide5f94ff6d2f75b845246d66a1c6554038bdc1f5