Add py36 testenv

Python 3.6 is installed by default in Ubuntu 18.04 LTS.
Therefore, according to Transition Plan [1],
it'll be handy to have py36 testenv.

For more details, please check Python2 Deprecation Timeline [2]
and Python3-first Goal - Completion Criteria [3].

[1] https://wiki.ubuntu.com/Python/Python36Transition
[2] https://governance.openstack.org/tc/resolutions/20180529-python2-deprecation-timeline.html
[3] https://review.openstack.org/#/c/575933/8/goals/stein/python3-first.rst@42

Change-Id: I1706900eb43d8189f1302d73bc469269b003e329

tox.ini

@@ -54,6 +54,11 @@ basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 
+[testenv:py36]
+basepython = python3.6
+deps = -r{toxinidir}/test-requirements.txt
+commands = ostestr {posargs}
+
 [testenv:pep8]
 basepython = python2.7
 deps = -r{toxinidir}/test-requirements.txt