[ussuri][goal] Cleanup for python 2.7 drop

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

puppet-* repos have to cleanup the requirement and tox
for py2.7 drop.

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: I2b249dd8bba18b0a1fe54a57284df346729c0be5