Only set basepython once

Rather than setting it in all of the envs, set it in the main env. Then
add ignore_basepython_conflict option to tox.ini so that doing so
doesn't mess with py35, py36 and py37's python setting.

Change-Id: I72fa337b2b7be1823beeb3f3bfc4bd23589c007d

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.8.1
+minversion = 3.1
 skipsdist = True
 envlist = pep8,py35
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 # Set STATSD env variables so that statsd code paths are tested.
 setenv = STATSD_HOST=localhost
          STATSD_PORT=8125
@@ -20,11 +22,9 @@ commands = stestr --test-path ./nodepool/tests/unit run --no-subunit-trace {posa
            stestr slowest
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 nodepool
 
 [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
@@ -33,7 +33,6 @@ deps = bindep
 commands = bindep test
 
 [testenv:cover]
-basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source nodepool --parallel-mode
@@ -44,18 +43,15 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -E -W -d doc/build/doctrees -b html doc/source/ doc/build/html
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:functional_kubernetes]
-basepython = python3
 commands = stestr --test-path ./nodepool/tests/functional/kubernetes run --no-subunit-trace {posargs}
            stestr slowest