We still have some Ubuntu Xenial servers, so cap the max usable pip
and setuptools versions in their venvs like we already do for
Bionic, in order to avoid broken installations. Switch our
conditionals from release name comparisons to version numbers in
order to more cleanly support ranges. Also make sure the borg run
test is triggered by changes to the create-venv role.
Change-Id: I5dd064c37786c47099bf2da66b907facb517c92a
2d9d24d07d