[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: Iebfc4ed12032f5b400ce8e7112f9d429285c5d36

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27,py34,py35
+envlist = pep8,py34,py35
 skipsdist = True
 # NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
 sitepackages = False
@@ -13,12 +13,6 @@ install_command =
   pip install {opts} {packages}
 commands = ostestr {posargs}
 
-[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