MAAS snap fails to parse supervisor STOPPING state

[Adam Collard]

Similar to bug 1871423

If a service is still running but asked to stop, it will enter STOPPING

http://supervisord.org/events.html#process-state-stopping-event-type

Found in system tests

2022-06-30 09:26:12 maasserver.region_controller: [critical] Failed configuring proxy.
        Traceback (most recent call last):
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in errback
            self._startRunCallbacks(fail)
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 568, in _startRunCallbacks
            self._runCallbacks()
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 654, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1475, in gotResult
            _inlineCallbacks(r, g, status)
        --- <exception caught here> ---
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/snap/maas/22346/lib/python3.8/site-packages/provisioningserver/utils/service_monitor.py", line 355, in restartService
            state = yield self.getServiceState(name, now=True)
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/snap/maas/22346/lib/python3.8/site-packages/provisioningserver/utils/service_monitor.py", line 282, in getServiceState
            active_state, process_state = yield self._loadServiceState(service)
          File "/snap/maas/22346/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
            result = g.send(result)
          File "/snap/maas/22346/lib/python3.8/site-packages/provisioningserver/utils/service_monitor.py", line 660, in _loadSupervisorServiceState
            raise ServiceParsingError(
        provisioningserver.utils.service_monitor.ServiceParsingError: Unable to parse the output from supervisor for service 'proxy'; supervisorctl returned status as 'STOPPING'