[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

ironic-tempest-plugin is ready with python 3 and ok to drop the
python 2.7 support.

All stable jobs need to run on py3 as master ironic-tempest-plugin
cannot support py2 anymore. Any py2 cloud/env can be tested by
installing the Temepest and plugins on py3 venv.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: Ibea78605aedb955e3dc70252ca380c7e5f8f8726
5 files changed