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: Id3804896efc11de24d5e13500cdce7571e6d3314
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -23,14 +23,17 @@ commands = stestr run {posargs}
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t neutron/tests {posargs}
 
 [testenv:common]
+basepython = python3
 # Fake job to define environment variables shared between dsvm/non-dsvm jobs
 setenv = OS_TEST_TIMEOUT={env: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
@@ -75,6 +78,7 @@ commands =
   stestr run {posargs}
 
 [testenv:dsvm-fullstack]
+basepython = python2.7
 setenv = {[testenv]setenv}
          {[testenv:common]setenv}
          {[testenv:dsvm]setenv}
@@ -97,6 +101,7 @@ commands =
   {[testenv:dsvm-fullstack]commands}
 
 [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
 
@@ -133,13 +138,16 @@ commands =
   coverage xml -o cover/coverage.xml
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:linkcheck]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b linkcheck doc/source doc/build/linkcheck
 
@@ -170,6 +178,7 @@ import_exceptions = neutron._i18n
 local-check-factory = neutron.hacking.checks.factory
 
 [testenv:bandit]
+basepython = python3
 # B104: Possible binding to all interfaces
 # B111: Execute with run_as_root=True identified, possible security issue
 # B311: Standard pseudo-random generators are not suitable for security/cryptographic purpose
@@ -177,6 +186,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r neutron -x tests -n5 -s B104,B111,B311
 
 [testenv:bashate]
+basepython = python3
 commands = bash -c "find {toxinidir}             \
          -not \( -type d -name .tox\* -prune \)  \
          -not \( -type d -name .venv\* -prune \) \
@@ -189,11 +199,13 @@ commands = bash -c "find {toxinidir}             \
          -print0 | xargs -0 bashate -v -iE006 -eE005,E042,E043"
 
 [testenv:genconfig]
+basepython = python3
 commands = {toxinidir}/tools/generate_config_file_samples.sh
 
 # This environment can be used to quickly validate that all needed system
 # packages required to successfully execute test targets are installed
 [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