Use subunit2html from tox envs instead of os-testr-env
We have an openstack-infra hard-coded location in this role. Rework it
to try looking in the tox env if one exists or to look on the system.
Leave /usr/os-testr-env in as a fallback but with a debug statement so
that we can track instances of the fallback being used.
tox_envlist can be a comma separated list. If it is, the "find
subunit2html in a tox env" logic isn't going to work.
Use a shell script so that we can do a first-found logic similar to what
we use in ensure-sphinx for finding requirements files.
It's a big bash blob. Put it in a file so it syntax highlights correctly
and execute it with the script module.
Change-Id: I0b468840fd04a379007fe5aca07feb2eaf7081ac
f6a92ecb6c