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: I77690fa682cc1b880d8b63ad5c17bd6bc2ee20ba

tox.ini

@@ -12,12 +12,14 @@ deps = bindep
 commands = bindep test
 
 [testenv:shellcheck]
+basepython = python3
 # 'shellcheck' is not an Python package, so it can be run w\o
 # virtual env. But tox is a usable wrapper to run any kind of tests -
 # let's use it for common test-run as well - for unification purposes.
 commands = {toxinidir}/tools/jenkins/shellcheck.sh {toxinidir}
 
 [testenv:yaml-syntaxcheck]
+basepython = python3
 deps = yamllint
 commands = {toxinidir}/tools/jenkins/yamllint.sh {toxinidir}
 
@@ -39,6 +41,7 @@ commands =
    {toxinidir}/tools/jenkins/yamllint.sh {toxinidir}
 
 [unit_testenv]
+basepython = python3
 # *unittest tox-test's *require* pre-installed and
 # configured murano-test-runner.
 # More info: http://docs.openstack.org/developer/murano/draft/appdev-guide/app_unit_tests.html
@@ -51,6 +54,7 @@ setenv =
     UPLOAD_TO_OS = true
 
 [testenv:refstackclient-unittest]
+basepython = python3
 passenv = OS_*
 setenv =
     {[unit_testenv]setenv}