Add py36 testenv
Add py36 testenv

Python 3.6 is installed by default in Ubuntu 18.04 LTS.
Therefore, according to transition plan [1],
we should have py36 testenv ready for it.

[1] https://wiki.ubuntu.com/Python/Python36Transition

Change-Id: I70c34207a37b7e4c3d917d6cbb6667c47495e860

stackforge/monasca-log-api,stackforge/monasca-log-api,openstack/monasca-log-api,openstack/monasca-log-api,openstack/monasca-log-api,stackforge/monasca-log-api