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: Ie537f11a4c2fe38a2fcec6d8a29a66c185101d6b

tox.ini

@@ -55,6 +55,17 @@ commands =
     rm -f .coverage
     rm -rf .testrepository
 
+[testenv:py36]
+basepython = python3.6
+setenv = OS_PATH_TEST = ./freezer_api/tests/unit
+commands =
+    find . -type f -name "*.py[c|o]" -delete
+    rm -rf .testrepository
+    python setup.py test --coverage --coverage-package-name freezer_api --testr-args="{posargs}"
+    coverage report -m
+    rm -f .coverage
+    rm -rf .testrepository
+
 [testenv:venv]
 commands = {posargs}