MAAS sends multiple stop_dhcp_server tasks even though there's no dhcp server running.

As seen here: http://d-jenkins.ubuntu-ci:8080/view/MAAS/job/utopic-adt-maas-manual/24/artifact/results/artifacts/maas-logs/var/log/maas/celery.log the stop_dhcp_server task is failing to stop the dhcp server.

A restart task triggered later on restarted the service. It seems that the MAAS server is trying to stop the dhcp server but fails because it wasn't running in the first place.

Package version: 1.6+bzr2367+2412+278-0ubuntu1

[2014-06-10 16:45:07,400: INFO/MainProcess] Received task: provisioningserver.tasks.stop_dhcp_server[27e5ebdb-66ed-4977-b0d5-de5e4c06e090]
[2014-06-10 16:45:08,055: ERROR/Worker-2] stop_dhcp_server: Command `sudo -n service maas-dhcp-server stop` returned non-zero exit status 1:
stop: Unknown instance:
[2014-06-10 16:45:08,063: ERROR/MainProcess] Task provisioningserver.tasks.stop_dhcp_server[27e5ebdb-66ed-4977-b0d5-de5e4c06e090] raised unexpected: ExternalProcessError()
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/celery/app/trace.py", line 240, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/celery/app/trace.py", line 437, in __protected_call__
    return self.run(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tasks.py", line 97, in wrapper
    func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/tasks.py", line 402, in stop_dhcp_server
    call_and_check(['sudo', '-n', 'service', 'maas-dhcp-server', 'stop'])
  File "/usr/lib/python2.7/dist-packages/provisioningserver/utils/__init__.py", line 160, in call_and_check
    raise ExternalProcessError(process.returncode, command, output=stderr)
ExternalProcessError