Use a local variable auditor_worker

This patch is only a cleanup, but our code makes me rage every time
I read it. In short, we have a class with a variable self.auditor,
and we assign ObjectAuditor to it at some ties, AuditorWorker at
other times. So, whenever there's a mismerge or whatever, you cannot
tell if self.auditor.broker_class makes sense or not.

Since all cases of using self.auditor as AuditorWorker are purely
local, just evict it to a local variable auditor_worker. Leave
self.auditor to represent ObjectAuditor, because it's used as
a class variable in places.

Change-Id: I811b80ac6c69a4daacfed7a5918bc0b15761d72e
Merge "Use a local variable auditor_worker"