Add tox-py38 and ensure that python interpreter is installed for tox-py3
jobs. The job includes the ensure-python role that checks and installs the
interpreter if python_version is set.
Test py37 to ensure that this works, it's the version currently used for
next OpenStack release, so let's test changes to roles and jobs here.
Run tox-py35 on Xenial, since that Xenial has Python 3.5 - it was run
previously on the default (3.6 in our install) instead, this change
showed that we run the wrong interpreter for tox-py35 on this repo.
Change-Id: I46d6780a5a32c443c37ef040b2af2459641dd123
25cc095e53