Add a source for Python 3.3 on Precise if needed.

* modules/jenkins/manifests/slave.pp: If $python3 is set add a
Python 3.3 PPA for Ubuntu 12.04 LTS, maintained by Chuck Short with
Canonical.

Change-Id: I6c6e2682e9c1d56698cfbce4f01c81b60eac674e
Reviewed-on: https://review.openstack.org/34078
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins