[ussuri][goal] Updates for python 2.7 drop

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

charm repos need few updates on either py2 drop or updating
ussuri python template, tox.ini etc.

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: I6c752f6302d0340dbecf2cb7fe4e2169139ea041

tox.ini

@@ -36,12 +36,6 @@ commands =
   /bin/ln -s {envdir}/tmp/build/builds {envdir}/tmp/build/trusty
   /bin/ln -s {envdir}/tmp/build {toxinidir}/build
 
-[testenv:py27]
-basepython = python2.7
-deps = -r{toxinidir}/test-requirements.txt
-# TODO: Need to write unit tests then remove the following command.
-commands = /bin/true
-
 [testenv:py34]
 basepython = python3.4
 deps = -r{toxinidir}/test-requirements.txt