Enable py36 unit tests in tox
Enable py36 unit tests in tox

With the current super-explicit version of tox.ini, it is not possible
to run tox -epy<any version> and have tests discovered and run. This
is because the default testenv does not run tests. Therefore, the
easiest way to add another version is to simply add another testenv,
as done in this change.

Python3.6 is the default Python3 on recent distributions so it needs
to be present if people are expected to be able to run python3 unit
tests before pushing their changes.

Change-Id: I728e482af3142bb101fc6bf52ec7926e29a42594

gooddata/openstack-nova,mikalstill/nova,gooddata/openstack-nova,klmitch/nova,gooddata/openstack-nova,klmitch/nova,phenoxim/nova,gooddata/openstack-nova,phenoxim/nova,mahak/nova,klmitch/nova,openstack/nova,mahak/nova,openstack/nova,rahulunair/nova,mikalstill/nova,rahulunair/nova,openstack/nova,rahulunair/nova,klmitch/nova,mahak/nova,mikalstill/nova