Add reno cmd in tox venv
Add reno cmd in tox venv

Now openstack can generate reno file by tox,
but there is an error in packstack when
using "tox -e venv -- reno new filename" for
there is no reno cmd in tox venv.
We need add the reno cmd to venv for supporting
the command above.

Change-Id: I3d76c787ce2f8fe143ed6693d3ef95ec3af50db5

stackforge/packstack,stackforge/packstack,openstack/packstack,fr34k8/packstack,openstack/packstack,openstack/packstack,fr34k8/packstack,fr34k8/packstack,stackforge/packstack,mahak/packstack,mahak/packstack,mahak/packstack