Allow multiple invocations of the same job

Previously if a job was listed more than once for a project, it was
ignored.  That's pretty arbitrary (it silently dropped the second
without an error; and if there are two, which is the right one
anyway?).  OTOH, it's potentially useful to run a job more than
once in order to increase the chance of encountering
notdeterministic behavior.  And if listing a job twice is an error,
it is now more likely to be noticed by the operator.

This removes the check for duplicate invocations of a job.

Change-Id: If8e2e8cc3fca855bd6b14eb3a957dadddfe143ed

zuul/model.py

@@ -532,10 +532,9 @@ class JobTree(object):
         self.job_trees = []
 
     def addJob(self, job):
-        if job not in [x.job for x in self.job_trees]:
-            t = JobTree(job)
-            self.job_trees.append(t)
-            return t
+        t = JobTree(job)
+        self.job_trees.append(t)
+        return t
 
     def getJobs(self):
         jobs = []