Fix bug with looking up a job tree.

If there are no jobs for a project in a pipeline, an exception
would be raised.  This change ensures that case is checked.

The test fixtures are updated to simulate this condition as well.

Change-Id: Id8e5bdef6288e5574d8553c11f5ac8e8c6c61704
Reviewed-on: https://review.openstack.org/10973
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: James E. Blair <corvus@inaugust.com>

tests/fixtures/layout.yaml

@@ -28,6 +28,13 @@ pipelines:
     start:
       verified: 0
 
+  - name: unused
+    manager: IndependentPipelineManager
+    trigger:
+      - event: comment-added
+        approval: 
+          - approved: 1
+
 jobs:
   - name: ^.*-merge$
     failure-message: Unable to merge change

zuul/model.py

@@ -23,6 +23,9 @@ class Pipeline(object):
         self.job_trees = {}  # project -> JobTree
         self.manager = None
 
+    def __repr__(self):
+        return '<Pipeline %s>' % self.name
+
     def setManager(self, manager):
         self.manager = manager
 
@@ -39,7 +42,7 @@ class Pipeline(object):
         return tree
 
     def getJobs(self, changeish):
-        tree = self.job_trees[changeish.project]
+        tree = self.getJobTree(changeish.project)
         if not tree:
             return []
         return changeish.filterJobs(tree.getJobs())
@@ -71,7 +74,7 @@ class Pipeline(object):
         return torun
 
     def findJobsToRun(self, changeish):
-        tree = self.job_trees[changeish.project]
+        tree = self.getJobTree(changeish.project)
         if not tree:
             return []
         return self._findJobsToRun(tree.job_trees, changeish)