Use python3 by default for testenvs

We need to be testing on python3 by default now. This switches the
docs, pep8, and genconfig testenvs to use python3.

Change-Id: I6d21e240e8b1c21698004fbe9e957476b7982c75

tox.ini

@@ -1,7 +1,8 @@
 [tox]
-minversion = 1.6
+minversion = 3.1
 skipsdist = True
 envlist = py36,py27,pep8
+ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
@@ -14,6 +15,7 @@ setenv =
 deps = -r{toxinidir}/test-requirements.txt
        -r{toxinidir}/requirements.txt
 commands = stestr run {posargs}
+basepython = python3
 
 [testenv:venv]
 commands = {posargs}
@@ -29,7 +31,6 @@ commands =
     bash -c bin/check-up-to-date.sh
 
 [testenv:cover]
-basepython = python3
 setenv =
     PYTHON=coverage run --source openstack_virtual_baremetal --parallel-mode
 commands =