tox.ini: Use python3.5 in py35 environment
tox.ini: Use python3.5 in py35 environment

When stuff fails it tends to fail in different ways on different
versions of python. It's very unhelpful if 'tox -epy35' means a
different thing on every developer's machine, which is what tends to
happen if you use python3 as the basepython.

Change-Id: I4f92cd6bb22e36b1c49436eca1849712682aec68

openstack/oslo.service