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: Idba5b407405e7d1457a02b074bf60b7fded284ee
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -22,6 +22,7 @@ commands =
   bash tools/pretty_tox.sh '{posargs}'
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t magnum/tests/unit {posargs}
 
 [testenv:debug-py27]
@@ -110,6 +111,7 @@ commands =
   bash tools/pretty_tox.sh '{posargs}'
 
 [testenv:pep8]
+basepython = python3
 commands =
   doc8 -e .rst specs/ doc/source/ contrib/ CONTRIBUTING.rst HACKING.rst README.rst
   bash tools/flake8wrap.sh {posargs}
@@ -123,24 +125,30 @@ commands =
       -print0 | xargs -0 bashate -v -iE006,E010,E042 -eE005"
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:bandit]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r magnum -x tests -n5 -ll
 
 [testenv:cover]
+basepython = python3
 commands = {toxinidir}/tools/cover.sh {posargs}
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:genconfig]
+basepython = python3
 commands =
     oslo-config-generator --config-file etc/magnum/magnum-config-generator.conf
 
 [testenv:genpolicy]
+basepython = python3
 commands =
     oslopolicy-sample-generator --config-file etc/magnum/magnum-policy-generator.conf
 
@@ -156,6 +164,7 @@ exclude = .venv,.git,.tox,dist,*lib/python*,*egg,build,tools,releasenotes
 local-check-factory = magnum.hacking.checks.factory
 
 [testenv:pip-check-reqs]
+basepython = python3
 # do not install test-requirements as that will pollute the virtualenv for
 # determining missing packages
 # this also means that pip_check_reqs must be installed separately, outside
@@ -165,15 +174,18 @@ deps = pip_check_reqs
 commands=pip-missing-reqs -d --ignore-file=magnum/tests/* magnum
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:install-guide]
+basepython = python3
 commands = sphinx-build -a -E -W -d install-guide/build/doctrees -b html install-guide/source install-guide/build/html
 
 [testenv:api-ref]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf api-ref/build
@@ -189,6 +201,7 @@ deps =
 # This environment can be used to quickly validate that all needed system
 # packages required to successfully execute test targets are installed
 [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