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: I3f2720a6e8e6c685d2e5ca288008be20785a8309

tox.ini

@@ -9,7 +9,6 @@ deps=
 passenv=HTTP_PROXY HTTPS_PROXY http_proxy https_proxy NO_PROXY no_proxy
 setenv=
     VIRTUAL_ENV={envdir}
-basepython = python3.5
 usedevelop = True
 install_command = pip install {opts} {packages}
 whitelist_externals =
@@ -27,24 +26,29 @@ commands =
     stestr slowest
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     rm -rf doc/build
     sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:genconfig]
+basepython = python3
 commands =
     oslo-config-generator --config-file=etc/armada/config-generator.conf
 
 [testenv:genpolicy]
+basepython = python3
 commands =
     oslopolicy-sample-generator --config-file=etc/armada/policy-generator.conf
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:pep8]
+basepython = python3
 deps =
     .[bandit]
     {[testenv]deps}
@@ -54,10 +58,12 @@ commands =
     bandit -r armada -x armada/tests -n 5
 
 [testenv:bandit]
+basepython = python3
 commands =
     bandit -r armada -x armada/tests -n 5
 
 [testenv:cover]
+basepython = python3
 setenv = {[testenv]setenv}
          PYTHON=coverage run --source armada --parallel-mode
 commands =