Join command thread on exit

The executor and fingergw miss joining the command thread. This could
lead to random test failures like [1].

[1] Trace:
Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/zuul/zuul/tests/base.py", line 4187, in shutdown
    raise Exception("More than one thread is running: %s" % threads)
Exception: More than one thread is running:
[<_MainThread(MainThread, started 140602311071488)>, <Thread(command, started daemon 140601232180992)>]

Change-Id: I5246b686fe708444ffaf9d94ef4321b304f1754e

zuul/executor/server.py

@@ -2801,6 +2801,7 @@ class ExecutorServer(BaseMergeServer):
         if self.process_merge_jobs:
             super().join()
         self.executor_gearworker.join()
+        self.command_thread.join()
 
     def pause(self):
         self.log.debug('Pausing')

zuul/lib/fingergw.py

@@ -217,3 +217,6 @@ class FingerGateway(object):
         Wait on the gateway to shutdown.
         '''
         self.server_thread.join()
+
+        if self.command_thread:
+            self.command_thread.join()