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.

NOTE: legacy-neutron-fwaas-dsvm-functional is failing.
This needs to be fixed in a separate patch.

Change-Id: I71dc6449c64b9a9f974d83e51423a7c67a42c551

tox.ini

@@ -27,6 +27,7 @@ setenv = OS_TEST_TIMEOUT=180
 commands = false
 
 [testenv:dsvm]
+basepython = python3
 # Fake job to define environment variables shared between dsvm jobs
 setenv = OS_SUDO_TESTING=1
          OS_ROOTWRAP_CMD=sudo {envdir}/bin/neutron-rootwrap {envdir}/etc/neutron/rootwrap.conf
@@ -36,6 +37,7 @@ setenv = OS_SUDO_TESTING=1
 commands = false
 
 [testenv:functional]
+#TODO(njohnston): Fix failures blocking dsvm-functional converting to py3
 setenv = {[testenv]setenv}
          {[testenv:common]setenv}
          OS_TEST_PATH=./neutron_fwaas/tests/functional
@@ -44,6 +46,7 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:dsvm-fullstack]
+basepython = python3
 setenv = {[testenv]setenv}
          {[testenv:common]setenv}
          {[testenv:dsvm]setenv}
@@ -53,6 +56,7 @@ setenv = {[testenv]setenv}
 sitepackages=True
 
 [testenv:api]
+basepython = python3
 sitepackages=True
 setenv =
  OS_TEST_PATH=./neutron_fwaas/tests/tempest_plugin/tests/api/
@@ -62,6 +66,7 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:scenario]
+basepython = python3
 sitepackages=True
 setenv =
  OS_TEST_PATH=./neutron_fwaas/tests/tempest_plugin/tests/scenario/
@@ -87,10 +92,12 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [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
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8
   doc8 {posargs}
@@ -101,11 +108,13 @@ commands =
 whitelist_externals = sh
 
 [testenv:cover]
+basepython = python3
 commands =
   python setup.py test --coverage --coverage-package-name=neutron_fwaas --testr-args='{posargs}'
   coverage report
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 deps = -r{toxinidir}/doc/requirements.txt
 
@@ -144,6 +153,7 @@ exclude = .venv,.git,.tox,dist,doc,*lib/python*,.tmp,*egg,build,tools,.ropeproje
 import-order-style = pep8
 
 [testenv:pylint]
+basepython = python3
 deps =
   {[testenv]deps}
   pylint
@@ -155,6 +165,7 @@ import_exceptions = neutron_fwaas._i18n
 local-check-factory = neutron_lib.hacking.checks.factory
 
 [testenv:genconfig]
+basepython = python3
 commands = {toxinidir}/tools/generate_config_file_samples.sh
 
 [testenv:genpolicy]