Centralize basepython as py3 in tox

OpenStack ussuri release will be python 3 only
so let's define the basepython in common env now.

Change-Id: I431523d0cd23c227e535c471bb0c74a3586fa696

tox.ini

@@ -1,15 +1,16 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
-basepython = python3
 deps =
      {[testenv]deps}
 commands =
@@ -19,11 +20,9 @@ commands =
 exclude =  .venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:status]
-basepython = python3
 whitelist_externals =
   mkdir
   bash