Propose 'tox' as tests launcher

Since ``tox`` is used as launcher in gates, it should be mentioned in docs
instead of ``python setup.py test``.

Change-Id: Ie5fcd9da755e1146b2b042fa5906aa28748980a6
Merge "Propose 'tox' as tests launcher"
Updated from global requirements

Change-Id: I3701c9964d442191de3a6581bcbc0b6f5e14ab26