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: Iddea984cd32fd0b048a396cde27e3d12ae07119e

tox.ini

@@ -20,9 +20,11 @@ commands =
   stestr slowest
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8
   # pylint --rcfile=.pylintrc --output-format=colorized {posargs:neutron_lbaas}
@@ -31,6 +33,7 @@ commands =
 whitelist_externals = sh
 
 [testenv:cover]
+basepython = python3
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source neutron_lbaas --parallel-mode
@@ -40,9 +43,11 @@ commands =
   coverage report
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [flake8]
@@ -77,6 +82,7 @@ deps =
   -r{toxinidir}/neutron_lbaas/tests/functional/requirements.txt
 
 [testenv:dsvm-functional]
+basepython = python3
 setenv = {[testenv]setenv}
          {[testenv:functional]setenv}
          OS_SUDO_TESTING=1
@@ -91,6 +97,7 @@ commands =
 # If you are running the tests locally you should set the env variable
 # TEMPEST_CONFIG_DIR=/opt/stack/tempest/etc
 [testenv:apiv2]
+basepython = python3
 passenv = TEMPEST_CONFIG_DIR
 setenv =
  OS_TEST_PATH={toxinidir}/neutron_lbaas/tests/tempest/v2/api
@@ -101,6 +108,7 @@ commands =
 # If you are running the tests locally you should set the env variable
 # TEMPEST_CONFIG_DIR=/opt/stack/tempest/etc
 [testenv:scenario]
+basepython = python3
 passenv = TEMPEST_CONFIG_DIR
 setenv =
  OS_TEST_PATH={toxinidir}/neutron_lbaas/tests/tempest/v2/scenario
@@ -111,6 +119,7 @@ commands =
 # If you are running the tests locally you should set the env variable
 # TEMPEST_CONFIG_DIR=/opt/stack/tempest/etc
 [testenv:ddt]
+basepython = python3
 passenv = TEMPEST_CONFIG_DIR
 setenv =
  OS_TEST_PATH={toxinidir}/neutron_lbaas/tests/tempest/v2/ddt
@@ -119,6 +128,7 @@ commands =
   stestr slowest
 
 [testenv:genconfig]
+basepython = python3
 commands = {toxinidir}/tools/generate_config_file_samples.sh
 
 [testenv:lower-constraints]