Add a testenv:py27 environment that overrides basepython to 2.7
Unfortunately implicit namespace packages are a Python3 thing [1] so
we have to scatter a few __init__.py's around for the test loader
under python2 to be able to find the unit test directories.
Update documenation to mention this
Needed-By: https://review.openstack.org/592768
[1] https://www.python.org/dev/peps/pep-0420/
Change-Id: I9a653666e8a083fb7f3fbb92589fe0467a41e6e6
Ian Wienand