Re-enable the py36 tox environment
Re-enable the py36 tox environment

A recent automated patch[1] removed py36 from the tox default
environment list. Since Ubuntu 18.04 includes python 3.6 and we
still include a functional-3.6 test, we should keep the py36
environment in the list.

[1] https://review.opendev.org/#/c/669440/

Change-Id: I2bab434523efee0ad755e794f00c3a1ca85ec68f

openstack/octavia,openstack/octavia,openstack/octavia