Simplify tarball and pypi publishing

Only publish distinct tarballs to tarballs.o.o for tags.
Use tarballs from tarballs.o.o when uploading to pypi instead of rebuilding.
Publish $project-$branch.tar.gz tarballs for every commit.

Change-Id: Ifd2c3b8c9734875ffb15528f84409f44e2f842da
Reviewed-on: https://review.openstack.org/18797
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins