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: Ia979ebfd5e98032b3f2a554017165348c5a3ca9c

tox.ini

@@ -44,6 +44,7 @@ commands =
 setenv = OS_FAIL_ON_MISSING_DEPS=1
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
@@ -74,6 +75,7 @@ deps =
 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]
@@ -86,6 +88,7 @@ exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,.ropeproject
 import-order-style = pep8
 
 [testenv:bandit]
+basepython = python3
 # B108 hardcoded_tmp_directory - remove when fixed
 # B602 subprocess_popen_with_shell_equals_true - removed when fixed
 # B605 start_process_with_a_shell - remove when fixed