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 of that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: I5e1239212b5a32c623cf00bf3cb0a56362a3e320

tox.ini

@@ -14,19 +14,24 @@ commands = python setup.py test --slowest --testr-args='{posargs}'
 whitelist_externals = bash
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
            bash -c tools/check_duplicate_jinja_blocks.sh
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py test --coverage --coverage-package-name=tripleo_common --testr-args='{posargs}'
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]
@@ -38,4 +43,5 @@ builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,releasenotes
 
 [testenv:releasenotes]
+basepython = python3
 commands = bash -c tools/releasenotes_tox.sh