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: I916dfe05c88d3960e0239a6b9a7e5affc6d79196
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -23,6 +23,7 @@ commands =
 commands = oslo_debug_helper -t kolla/tests {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          NOSE_WITH_COVERAGE=1
          NOSE_COVER_BRANCHES=1
@@ -32,6 +33,7 @@ commands = python setup.py testr --coverage --testr-args='{posargs}'
            coverage report --show-missing
 
 [testenv:pep8]
+basepython = python3
 deps =
     {[testenv]deps}
     yamllint
@@ -48,9 +50,11 @@ commands =
 commands = bandit --skip B303 -r docker kolla tests tools
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/requirements.txt
@@ -243,12 +247,14 @@ commands=
   oslo-config-generator --config-file etc/oslo-config-generator/kolla-build.conf
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:bindep]
+basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed