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 of that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: I38db902c553d25f528b3ba9a80e185705f327b24

tox.ini

@@ -16,6 +16,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands =
   sh ./tools/misc-sanity-checks.sh
   flake8
@@ -23,9 +24,11 @@ whitelist_externals =
   sh
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source neutron_tempest_plugin --parallel-mode
@@ -36,13 +39,16 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
+basepython = python3
 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
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t neutron_tempest_plugin/ {posargs}
 
 [flake8]