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: Ica8253d82288ecf2e281b521b7ade989c8d6c442

tox.ini

@@ -15,17 +15,21 @@ commands =
          stestr run {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands =
    python setup.py build_sphinx {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands =
    flake8 {toxinidir}/openstack_health {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
    {[testenv]setenv}
    PYTHON = coverage run --source openstack_health --parallel-mode
@@ -45,7 +49,9 @@ show-source = True
 exclude = .git,.venv,.tox,dist,doc,openstack,*egg
 
 [testenv:npm-test]
+basepython = python3
 commands = npm test
 
 [testenv:npm-lint]
+basepython = python3
 commands = npm run lint --silent