I created a tox PR[1] to add native support for constraints. The answer
was basically "use deps".
I tested it locally with shade and tox and it does indeed seem to do
what we want. Use shade as a guinea pig before rolling this out wider.
[1] https://github.com/tox-dev/tox/pull/642
Change-Id: I450dd513d7e1f881db764b11dcd6f9072c0c369c
Depends-On: Id61ae52d48b28cfc2221cb556a1c1f7c6dfd60dd
shade is in global requirements and upper-constraints now, which means
we need to use the tox_install.sh script so that constraints don't break
install of shade itself.
Change-Id: Id286b634046519e34ac2c01461391d5bc61cd919