Don't ignore inexistent jobs in config

Currently, when a project pipeline invokes a job which otherwise
doesn't exist, zuul simply runs nothing. As this is not the desired
behavior add a check during configuration time.

This also uncovers some bugs in the tests which are also fixed.

Change-Id: I37da435fdf3281f98ca097d79d504c170c13bf3a
Story: 2000893
4 files changed