[bug] Management tunnel goes down just before iperf3 check was executed

[Aryamanz29]

Reported by @pandafy in #394 (review)

• The database reports that DeviceConnection is working, but the management tunnel goes down just before iperf check was executed. The code should be able to gracefully handle this and also notify users that SSH connection is not working.

Error log

[2022-06-21 14:04:53,311: ERROR/ForkPoolWorker-8] 'NoneType' object has no attribute 'open_session'
Traceback (most recent call last):
  File "/home/aryaman/Desktop/Iperf-GSoC/env/lib/python3.8/site-packages/openwisp_controller/connection/connectors/ssh.py", line 174, in exec_command
    stdin, stdout, stderr = self.shell.exec_command(command, timeout=timeout)
  File "/home/aryaman/Desktop/Iperf-GSoC/env/lib/python3.8/site-packages/paramiko/client.py", line 508, in exec_command
    chan = self._transport.open_session(timeout=timeout)
AttributeError: 'NoneType' object has no attribute 'open_session'
[2022-06-21 14:04:53,313: ERROR/ForkPoolWorker-8] Task openwisp_monitoring.check.tasks.perform_check[9b3fe0a6-e740-4bcc-abce-7c616c4837e7] raised unexpected: AttributeError("'NoneType' object has no attribute 'open_session'")
Traceback (most recent call last):
  File "/home/aryaman/Desktop/Iperf-GSoC/env/lib/python3.8/site-packages/celery/app/trace.py", line 451, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/home/aryaman/Desktop/Iperf-GSoC/env/lib/python3.8/site-packages/celery/app/trace.py", line 734, in __protected_call__
    return self.run(*args, **kwargs)
  File "/home/aryaman/Desktop/Iperf-GSoC/openwisp-monitoring/openwisp_monitoring/check/tasks.py", line 48, in perform_check
    result = check.perform_check()
  File "/home/aryaman/Desktop/Iperf-GSoC/openwisp-monitoring/openwisp_monitoring/check/base/models.py", line 86, in perform_check
    return self.check_instance.check(store=True)
  File "/home/aryaman/Desktop/Iperf-GSoC/openwisp-monitoring/openwisp_monitoring/check/classes/iperf.py", line 37, in check
    res, exit_code = device_connection.connector_instance.exec_command(
  File "/home/aryaman/Desktop/Iperf-GSoC/env/lib/python3.8/site-packages/openwisp_controller/connection/connectors/ssh.py", line 182, in exec_command
    raise e
  File "/home/aryaman/Desktop/Iperf-GSoC/env/lib/python3.8/site-packages/openwisp_controller/connection/connectors/ssh.py", line 174, in exec_command
    stdin, stdout, stderr = self.shell.exec_command(command, timeout=timeout)
  File "/home/aryaman/Desktop/Iperf-GSoC/env/lib/python3.8/site-packages/paramiko/client.py", line 508, in exec_command
    chan = self._transport.open_session(timeout=timeout)
AttributeError: 'NoneType' object has no attribute 'open_session'