[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: I46cff2dc02fb16db35d01fe6454ee67732d8def5

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27
+envlist = pep8,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:py35]
 basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt