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: I5f7093b6b8a7c10e519d5b56a168f4ac4fb2560e

tox.ini

@@ -32,6 +32,7 @@ setenv =
 
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands=
     bash -c "rm -rf doc/build"
@@ -45,6 +46,7 @@ extensions = .rst
 
 
 [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
@@ -52,11 +54,13 @@ commands =
 
 # environment used by the -infra templated docs job
 [testenv:venv]
+basepython = python3
 commands =
     {posargs}
 
 
 [testenv:pep8]
+basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-pep8.sh"
 
@@ -69,6 +73,7 @@ ignore=F403
 
 
 [testenv:bashate]
+basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-bashate.sh"
 
@@ -95,6 +100,7 @@ commands =
 
 
 [testenv:functional]
+basepython = python3
 deps =
     {[testenv:ansible]deps}
 commands =
@@ -102,6 +108,7 @@ commands =
 
 
 [testenv:ssl]
+basepython = python3
 deps =
     {[testenv:ansible]deps}
 setenv =
@@ -112,6 +119,7 @@ commands =
 
 
 [testenv:linters]
+basepython = python3
 deps =
     {[testenv:ansible]deps}
 commands =