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.
Fix unit tests after underlying changes in charms.openstack which
set_config_defined_certs_and_keys and added oncigure_source to
the default install method.
Change-Id: I82445240c49a2d58c6eb39da4f49d21350d8d8b4
Partial-Bug: 1642981
91d8d10382