Explicitly use python2.7 for linters and basic jobs

Queens is the last version before the introduction of the new PTI
which switched the basic jobs to python3.
Despite being EM, the pep8 job is failing because it tries to use
python3 (now the default on the OpenDev-provided Zuul instances)
but it is still worth fixing it.

Change-Id: Ifa03268f3e062ee8e3b4453b48163cd5e504451d

tox.ini

@@ -24,9 +24,11 @@ commands = find . -type f -name "*.pyc" -delete
 whitelist_externals = find
 
 [testenv:pep8]
+basepython = python2.7
 commands = flake8
 
 [testenv:pylint]
+basepython = python2.7
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/queens}
   -r{toxinidir}/requirements.txt
@@ -62,6 +64,7 @@ deps =
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:functional]
+basepython = python2.7
 commands = ostestr {posargs}
 setenv =
   {[testenv]setenv}