fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: Ie3d915e6a566e3503a7e1cb8bd1543626edb0af6

tox.ini

@@ -12,25 +12,31 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:stress]
+basepython = python3
 envdir = .tox/tempest_stress
 sitepackages = False
 commands =
     run-tempest-stress {posargs}
 
 [testenv:pep8]
+basepython = python3
 sitepackages = False
 commands = flake8 {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]