Revert "Allow multiple invocations of the same job"

This reverts commit 3aa372722546252f629ad996ce206ece5ab7e582.

There are several more places in Zuul where it is assumed that job
names are unique.  Fixing them all would be complicated and perhaps
not worth the benefit.  Go back to requiring that they be unique for
a project.

Change-Id: I5b8aa33d6aed9cb086f44594ce59947c0190cc11