publish per branch constraints files to tarballs.o.o

[Matthew Thode]

1. Add a post job to all branches that publish a constraints/$series.txt to $series
  * On the master branch we publish both master.txt and $series.txt (currently queens.txt)
  * when we fork rocky from master we update the publish job to publish master and rocky.txt.
  * As Doug points out there is a timing race here but it's much smaller than the one we have now.

2. We update the projects to use the static (non-git) URL for the constraints files.

3. We update the release tools to generate the new style URL.
  * Optionally, and this requires discussion, we use a custom tox_install.sh to extract the branch from .gitreview and generate the URL which would remove the need for the last step.

[Matthew Thode (prometheanfire)]

this was copied from our etherpad