Run all jobs by default using python3

This patch implements the community wide goal to run all jobs by
default using python3.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: I4e62491ac9a9996cfd910e7d69897b92072bb1eb

.zuul.yaml

@@ -1,5 +1,7 @@
 - project:
     templates:
-      - openstack-python-jobs
-      - openstack-python35-jobs-nonvoting
       - check-requirements
+      - openstack-python-jobs
+      - openstack-python35-jobs
+      - openstack-python36-jobs
+      - release-notes-jobs-python3

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py27,pep8
+envlist = pep8,py27,py35,py36
 minversion = 2.0
 skipsdist = True
 
@@ -23,6 +23,21 @@ commands = python manage.py test {posargs} --settings=masakaridashboard.test.set
 basepython = python3
 commands = flake8 {posargs}
 
+[testenv:py27]
+basepython = python2.7
+commands =
+   {[testenv]commands}
+
+[testenv:py35]
+basepython = python3.5
+commands =
+   {[testenv]commands}
+
+[testenv:py36]
+basepython = python3.6
+commands =
+    {[testenv]commands}
+
 [testenv:venv]
 basepython = python3
 commands = {posargs}