Add branch tarballs for tuskar projects

The tuskar projects have requested automatic creation of branch
tarballs.

* .../files/zuul/layout.yaml: Add .*-branch-tarball jobs in the post
pipeline for the python-tuskarclient, tuskar and tuskar-ui projects.

Change-Id: I2234e70376edc1c9483f984a44da24391353deb1
Closes-Bug: #1234358
Merge "Add branch tarballs for tuskar projects"