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