[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: I860b169528a7130a6c755ad494f6b3db08ea6aee

.zuul.yaml

@@ -1,5 +1,5 @@
 - project:
     templates:
       - python35-charm-jobs
-      - openstack-python3-train-jobs
+      - openstack-python3-ussuri-jobs
       - openstack-cover-jobs

tox.ini

@@ -20,14 +20,6 @@ passenv = http_proxy https_proxy
 install_command =
   pip install {opts} {packages}
 
-[testenv:py27]
-basepython = python2.7
-# Reactive source charms are Python3-only, but a py27 unit test target
-# is required by OpenStack Governance.  Remove this shim as soon as
-# permitted.  http://governance.openstack.org/reference/cti/python_cti.html
-whitelist_externals = true
-commands = true
-
 [testenv:py35]
 basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt