Validate that zuul-referenced jobs exist

Create a list of jobs defined by the current JJB configuration
and use it to validate that every job referenced in the Zuul
layout.yaml exists.

Also, fix all of the errors currently identified by this.

Requires Iceb74440cb004e9ebe6fc08a4eedf7715de2d485 in Zuul.

Change-Id: Iae8f612836a269520ce7448b9a656c8df183e126
Merge "Validate that zuul-referenced jobs exist"