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: I4cf01eedf29fd9406722f32838920f2af1704a08
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -27,6 +27,7 @@ basepython = python2.7
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY REQUESTS_CA_BUNDLE
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 distribute = false
 
@@ -47,6 +48,7 @@ basepython = python3.7
 
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:debug]
@@ -67,6 +69,7 @@ commands =
   {toxinidir}/tests/ci/rally_functional_job.sh {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = {toxinidir}/tests/ci/cover.sh {posargs}
 
 
@@ -91,6 +94,7 @@ exclude=.venv,.git,.tox,dist,*lib/python*,*egg,tools,build,setup.py
 local-check-factory = tests.hacking.checks.factory
 
 [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