[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: Ic9587fda5e2ba04ef151c32706b8da04a04096d4

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}
@@ -30,7 +32,6 @@ setenv =
     WORKING_DIR={toxinidir}
 
 [testenv:docs]
-basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     bash -c "rm -rf doc/build"
@@ -38,7 +39,6 @@ commands =
     sphinx-build --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-basepython = python3
 deps = {[testenv:docs]deps}
 whitelist_externals =
   make
@@ -51,19 +51,16 @@ 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"
 
@@ -75,17 +72,14 @@ commands =
 ignore=F403,H303
 
 [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"
 
@@ -94,7 +88,6 @@ 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}