Be explicit about tox python version
Be explicit about tox python version

We want to default all jobs to use python 3 except for
the ones that use explicit versions. Unfortunately, the
current tox behavior for setting basepython to python3
is not intuitive.

If the system default is python 3.5, then when basepython
is set to python3, both the py35 and py36 targets will
end up running with python 3.5.

To get around this we need to not set a default base
version and explicitly set each testenv to python3 that
we want to run using 3 instead of 2 but do not really
care which minor version is used.

Change-Id: I3971eeafbd1e732a584b0c52d2eb6c2175b709aa

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder