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: I652ae850c2cb8de7103cfe41caa861751893e9da
Partial-Bug: 1642981

tox.ini

@@ -24,6 +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
@@ -35,7 +43,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 
 [testenv:pep8]
-basepython = python2.7
+basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} src unit_tests