Fix graceful exit

When the gearman server was added, the exit handler was not updated
correctly.  It should tell the scheduler to exit, wait for the
scheduler to empty its pipelines, and then kill the gearman server
and exit.

Change-Id: Ie0532c2ea058ed56217e41641f8eec45080a9470

zuul/cmd/server.py

@@ -103,8 +103,9 @@ class Server(object):
 
     def exit_handler(self, signum, frame):
         signal.signal(signal.SIGUSR1, signal.SIG_IGN)
-        self.stop_gear_server()
         self.sched.exit()
+        self.sched.join()
+        self.stop_gear_server()
 
     def term_handler(self, signum, frame):
         self.stop_gear_server()