[infra][zuul][kolla] ansible_python_interpreter & stable branches

Hi, It appears that Zuul is now using python3 as the ansible_python_interpreter for Ubuntu [0]. This seems to also affect stable branches. For kolla ansible this has broken our deploy job which uses the pip module to install our dependencies, and now does so using python3. For Stein and earlier, our scripts use #/usr/bin/env python, which links to python2. I think we can easily fix this by setting ansible_python_interpreter in our zuul vars on stable branches, but perhaps there are some other unintended side-effects? Cheers, Mark [0] https://zuul.opendev.org/t/openstack/build/e0c56c36ef064c9dae00d31ea2979b32/...