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: I3c8e5c663a3fd881430f84fc502e46192de77fc7

tox.ini

@@ -16,6 +16,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 
 [testenv:functional]
+basepython = python3
 usedevelop = True
 # Define virtualenv directory, port to use for functional testing, and number
 # of seconds to wait for the agent to come alive during functional testing.
@@ -28,6 +29,7 @@ setenv =
 commands = ostestr {posargs}
 
 [testenv:pep8]
+basepython = python3
 whitelist_externals = bash
 commands =
   flake8 {posargs:ironic_python_agent imagebuild}
@@ -37,6 +39,7 @@ commands =
   doc8 doc/source README.rst
 
 [testenv:cover]
+basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          PYTHON=coverage run --source ironic_python_agent --omit='*tests*' --parallel-mode
 commands =
@@ -47,24 +50,29 @@ commands =
   coverage html -d ./cover --omit='*tests*'
 
 [testenv:venv]
+basepython = python3
 setenv = PYTHONHASHSEED=0
 commands = {posargs:}
 
 [testenv:debug]
+basepython = python3
 envdir = {toxworkdir}/venv
 commands = oslo_debug_helper -t ironic_python_agent/tests/unit {posargs}
 
 [testenv:devenv]
+basepython = python3
 envdir = devenv
 usedevelop = True
 
 [testenv:docs]
+basepython = python3
 setenv = PYTHONHASHSEED=0
 sitepackages = False
 commands =
   python setup.py build_sphinx
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [flake8]
@@ -86,12 +94,14 @@ enable-extensions=H106,H203,H204,H205,H210,H904
 import_exceptions = ironic.openstack.common.gettextutils._,testtools.matchers
 
 [testenv:checkconfig]
+basepython = python3
 sitepackages = False
 envdir = {toxworkdir}/venv
 commands =
   {toxinidir}/tools/config/check_uptodate.sh
 
 [testenv:genconfig]
+basepython = python3
 sitepackages = False
 envdir = {toxworkdir}/venv
 commands =