Update tox.ini for py27 requirement

Reactive source charms are Python3-only, but have py27 unit tests
declared in project-config.

The Tox tool recently changed behavior.  It used to pass when a tox
target was missing commands.  Now it fails in that case.

This commit places a py27 no-op shim to allow gate tests to pass,
effectively restoring the original behavior for these py3x-only repos.

Change-Id: Ia3e7be0bf9ac085c66ce318f283f42b0c7cc73fe
Partial-Bug: 1642981

tox.ini

@@ -1,3 +1,6 @@
+# Source charm: ./tox.ini
+# This file is managed centrally by release-tools and should not be modified
+# within individual charm repos.
 [tox]
 skipsdist = True
 envlist = pep8,py34,py35
@@ -21,6 +24,14 @@ basepython = python2.7
 commands =
     charm-build --log-level DEBUG -o {toxinidir}/build src {posargs}
 
+[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:py34]
 basepython = python3.4
 deps = -r{toxinidir}/test-requirements.txt
@@ -32,7 +43,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 
 [testenv:pep8]
-basepython = python3.4
+basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} src unit_tests