Add tarball site to packstack jobs.

This is required for tarball related jobs in python-jobs.

Change-Id: If8c33556a0f93ff69674b3cfc69ead091c7f2aff
Reviewed-on: https://review.openstack.org/19866
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins