fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in [testenv].

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 must explicitly set basepython in envs that are usually "automatic" -
otherwise due to https://github.com/tox-dev/tox/issues/425 these will
use `basepython` instead of their automatic equivalents.

Change-Id: I1026110f0f1736127cf0af8ec2ef791a0f999e3a
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -4,6 +4,7 @@ envlist = py{35,27},functional,pep8,pip-missing-reqs
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 whitelist_externals =
   bash
@@ -26,6 +27,8 @@ passenv = OS_DEBUG GENERATE_HASHES
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 
 [testenv:py27]
+# TODO(efried): Remove this once https://github.com/tox-dev/tox/issues/425 is fixed.
+basepython = python2.7
 commands =
   {[testenv]commands}
   stestr run {posargs}
@@ -33,20 +36,22 @@ commands =
   stestr slowest
 
 [testenv:py35]
+# TODO(efried): Remove this once https://github.com/tox-dev/tox/issues/425 is fixed.
+basepython = python3.5
 commands =
   {[testenv]commands}
   stestr run --blacklist-file=tests-py3.txt {posargs}
   env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
 
 [testenv:py36]
+# TODO(efried): Remove this once https://github.com/tox-dev/tox/issues/425 is fixed.
+basepython = python3.6
 commands =
   {[testenv]commands}
   stestr run --blacklist-file=tests-py3.txt {posargs}
   env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
 
 [testenv:pep8]
-# Let's gate pep8 under py3 by default because the py3 checks are stricter.
-basepython = python3
 deps = {[testenv]deps}
 commands =
   bash tools/flake8wrap.sh {posargs}
@@ -56,8 +61,6 @@ commands =
   bash -c '! find doc/ -type f -name *.json | xargs -t -n1 python -m json.tool 2>&1 > /dev/null | grep -B1 -v ^python'
 
 [testenv:fast8]
-# Let's run fast8 under py3 by default because the py3 checks are stricter.
-basepython = python3
 # This is a subset of the full pep8 check which
 # only runs flake8 on the changes made since
 # HEAD-1. For a full flake8 run including checking
@@ -234,7 +237,6 @@ commands = bindep test
 usedevelop = False
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt