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: I3639551e0e8d0b05c7d76158b91771294d66a2c9

tox.ini

@@ -13,25 +13,31 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
+basepython = python3
 commands =
     doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
     flake8
     bandit -r watcher_tempest_plugin -x tests -n5 -ll -s B320
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [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 {posargs}
 
 [flake8]