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: I6941acb53a22f434947f8b40e6f86b8c14b57d5d
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -27,26 +27,32 @@ commands =
   python setup.py test --slowest --testr-args="--concurrency=4 {posargs}"
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
   python setup.py test --coverage --testr-args='{posargs}'
   coverage report
 
 [testenv:docs]
+basepython = python3
 whitelist_externals = rm
 commands = rm -rf doc/build
            rm -rf doc/source/contributor/api
            python setup.py build_sphinx
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t karbor/tests/unit {posargs}
 
 [testenv:api-ref]
+basepython = python3
 # This environment is called from CI scripts to test and publish
 # the API Ref to developer.openstack.org.
 whitelist_externals = rm
@@ -56,12 +62,15 @@ commands =
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html/
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:genconfig]
+basepython = python3
 commands = oslo-config-generator --config-file etc/oslo-config-generator/karbor.conf
 
 [testenv:genpolicy]
+basepython = python3
 commands = oslopolicy-sample-generator --config-file=etc/karbor-policy-generator.conf
 
 [flake8]