Handle task manager shutdown more correctly

If a task manager was stopped with tasks in queue, they would
not be processed.  This could result in deadlocked threads if
one of the long-running threads was waiting on a task while
the main thread shut down a manager.

Change-Id: I3f58c599d472d134984e63b41e9d493be9e9d70b

nodepool/task_manager.py

@@ -71,9 +71,11 @@ class TaskManager(threading.Thread):
 
     def run(self):
         last_ts = 0
-        while self._running:
+        while True:
             task = self.queue.get()
             if not task:
+                if not self._running:
+                    break
                 continue
             while True:
                 delta = time.time() - last_ts