Import the python-tarball playbook from openstack-zuul-jobs and call
it pti-python-tarball to reflect the fact that it honors the OpenStack
Python Testing Interface.
Redefine release-openstack-python3 to use the new pti-python-tarball
playbook instead of inheriting from release-openstack-python, because
the old job uses the old python-tarball playbook which does not honor
the release_python variable.
Change-Id: Ic6c1da3e05d3fc59700ed97e1e5aafa3196d9fb6
Signed-off-by: Doug Hellmann <doug@doughellmann.com>