Code review comment for lp:~canonical-platform-qa/autopilot/cpo-base-classes-fix

I've uploaded some fixes for errors spotted in the gatekeeper jobs. These were related to the breadth-first nature of method resolution: given one class that doesn't have a validate_dbus_object method of its own and that method is inherited from a non-direct ancestor, when you insert in its bases an extension class with that method defined then that inserted method have preference over the inherited one.

The solution adopted is removing the validate_dbus_object method from the extension classes being added to the bases of the CPOs, so that the correct method resolution is preserved. This has given good results in gatekeeper runs with partial suites, currently there's a complete run being executed (job #271).