Adding auth_version option to zuul.conf

Zuul swift plugin checks for auth_version in zuul.conf. If swift
not found this option, it assumes 2.0 as default.
This patch add the possibility to specify which version you want
to use, allowing custom versions. It will be good in future when
version 3.0 releases.

Change-Id: I1ffd327feba8d290bf05291b022fff00af454c48
Merge "Adding auth_version option to zuul.conf"