tests: allow passing posargs to py.test
tests: allow passing posargs to py.test

This allows to select individual unit tests when running tox, such as:

    tox -e py3-unit -- -k patch

will only run unit tests with 'patch' in the name.

Change-Id: I151deca322be59a4155ebf85da5db30c928fea45

redhat-openstack/rdopkg,openstack-packages/rdopkg,redhat-openstack/rdopkg,openstack-packages/rdopkg