We need to generate constraints for all python versions we use. This is
python 2.7 (trusty and xenial), 3.4 (trusty) and 3.5(xenial). There
isn't a supportable way to install all 3 versions on a single node.
Add a version-map that allows us to clone the output of a freeze between
python versions. This assumes that the python version in question are
compatible during the transition window.
It's possible to run with:
... --version-map 3.4:3.5 --version-map 3.5:3.4 ...
This will mean that we generate constraints consistently between versions
regardless of which version /usr/bin/python3 is.
Change-Id: I55a3597ea2f2bf48e30dc564a4bf4b2ef7f178b3
ffe658de93