[ussuri][goal] Updates for python 2.7 drop

OpenStack is dropping the py2.7 support in ussuri cycle.

Only update required is in tox to define common baspython as py3 os that
all tox env including [testenv:functional] will use py3 instead of py2.

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: I1afb7ef6ef2cced0be9d4c148adbcb873c5dfec1

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 skipsdist = True
 envlist = docs,linters,functional
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command =
     pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
@@ -31,7 +33,6 @@ setenv =
     WORKING_DIR={toxinidir}
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands=
     bash -c "rm -rf doc/build"
@@ -43,19 +44,16 @@ commands=
 extensions = .rst
 
 [testenv:releasenotes]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 # environment used by the -infra templated docs job
 [testenv:venv]
-basepython = python3
 commands =
     {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-pep8.sh"
 
@@ -66,17 +64,14 @@ commands =
 ignore=F403
 
 [testenv:bashate]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-bashate.sh"
 
 [testenv:ansible-syntax]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-syntax.sh"
 
 [testenv:ansible-lint]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-lint.sh"
 
@@ -85,7 +80,6 @@ commands =
     bash -c "{toxinidir}/tests/common/test-ansible-functional.sh"
 
 [testenv:linters]
-basepython = python3
 commands =
     bash -c "{toxinidir}/tests/common/test-ansible-env-prep.sh"
     {[testenv:pep8]commands}