Stackforge documentation example corrections.

* doc/source/stackforge.rst: Lately we'd rather test on quantal
slaves than precise. Also, the python-jobs group includes some
tarball job templates, which will cause JJB to raise a keyerror
exception if tarball-site isn't set.

Change-Id: I282e1bf059210bf13c7c30de1fad0b1be43ae0a8
Reviewed-on: https://review.openstack.org/25172
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins