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: I0637976fc3097ff5052e38cf8a9afc7a3330034f
Depends-On: I2cbf2e63a9d93c232b6928acf002a45b7fbcec8e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -19,23 +19,28 @@ commands = find . -type f -name "*.pyc" -delete
 whitelist_externals = find
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8
   bandit -r keystoneclient -x tests -n5
 
 [testenv:bandit]
+basepython = python3
 # NOTE(browne): This is required for the integration test job of the bandit
 # project. Please do not remove.
 commands = bandit -r keystoneclient -x tests -n5
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py testr --coverage --testr-args='{posargs}'
            coverage report
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t keystoneclient/tests {posargs}
 
 [testenv:functional]
@@ -55,10 +60,12 @@ show-source = True
 exclude = .venv,.tox,dist,doc,*egg,build
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 deps = -r{toxinidir}/doc/requirements.txt
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 deps = -r{toxinidir}/doc/requirements.txt
 
@@ -67,6 +74,7 @@ import_exceptions =
     keystoneclient.i18n
 
 [testenv:bindep]
+basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed