Add a tactical change which is already merged into charm-helpers.

This needs to go into all charms to solve the chicken:egg issue
where cosmic is untestable until this change exists.

Reference:

https://github.com/juju/charm-helpers/commit/4835c6c167c429527ef0a0291d17cf559c9cf880

Change-Id: I7c775abf0172bb17e047542c8b1fc728c56bdfe5