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: I6b089c76f6a7aae9e4ad10e4429227523d47ddb9

tox.ini

@@ -18,14 +18,17 @@ commands = ostestr {posargs}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
+basepython = python3
 commands =
     flake8 {posargs}
     doc8 README.rst CONTRIBUTING.rst HACKING.rst doc/source
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv = {[testenv]setenv}
          PYTHON=coverage run --source ironic_staging_drivers --omit='*tests*' --parallel-mode
 commands =
@@ -36,12 +39,15 @@ commands =
     coverage html -d ./cover --omit='*tests*'
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]