[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

@@ -8,7 +8,6 @@ sitepackages = False
 skip_missing_interpreters = False
 
 [testenv]
-basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
          TERM=linux
@@ -25,6 +24,7 @@ whitelist_externals = /bin/true /bin/echo /bin/mkdir /bin/ln
 # The gate check will look for and validate the built artifacts in the source
 # tree.
 # Build the artifats under /tmp and link back to source directory to alleviate.
+basepython = python3
 commands =
   /bin/echo 'WARNING: *build* target is for testing only.'
   /bin/mkdir -p {envdir}/tmp
@@ -33,14 +33,9 @@ commands =
   /bin/ln -s {envdir}/tmp/build {toxinidir}/build
 
 [testenv:venv]
+basepython = python3
 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