Train, Stein & Rocky only
Zuul seems to have started setting ansible_python_interpreter to python3
on Ubuntu, which breaks our installation of kolla-ansible on Train and
earlier stable branches.
This change sets ansible_python_interpreter explicitly to python2. We
use host-vars rather than vars because zuul sets a host-var for
ansible_python_interpreter=auto, which overrides vars.
Change-Id: I9ba0d16972b31a64617461461fafaf2b436e2bdf
d30eacff3e