[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: Id04e5bae7f096a5b02225e9def1d7d25731b057e

tox.ini

@@ -1,4 +1,5 @@
 [tox]
+minversion = 3.1
 skipsdist = True
 envlist = pep8
 toxworkdir = /tmp/tox
@@ -6,6 +7,7 @@ toxworkdir = /tmp/tox
 sitepackages = False
 # NOTE(beisner): Avoid false positives by not skipping missing interpreters.
 skip_missing_interpreters = False
+ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
@@ -35,12 +37,6 @@ commands =
 [testenv:venv]
 commands = {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
@@ -66,5 +62,4 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = /bin/true
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 --ignore=E402 actions/ reactive/