Shut down gearman client in tests

Some tests spin up a gearman client to check on job status.  That
client sticks acound forever and continues to run in subsequent
tests.

Shut it down after use.

Change-Id: I49fcad20a6bf185d27a1036dfd0d1a5cc7615307

nodepool/tests/__init__.py

@@ -527,6 +527,7 @@ class DBTestCase(BaseTestCase):
 
         while client.get_queued_image_jobs() > 0:
             time.sleep(.2)
+        client.shutdown()
 
     def useNodepool(self, *args, **kwargs):
         args = (self.secure_conf,) + args