Custom Build Complete Actions not running for jobs

The BuildCompletedEventListener only runs CustomBuildCompletActions against Build objects. So plugins will never run against jobs. Strangely there is todo in the class that says it should run for top level plans.

We need to
a) confirm what it should be running and ensure I am not dreaming that it doesn't work
b) Fix it and put in on 2.7 final

This issue is going to effect plugins, and prevents the auto build labeller from working at all.