Fix jstesnode software release test random failure

Fix this random failure we sometimes have, like [here](https://nexedijs.erp5.net/#/test_result_module/20200414-44DF7D23/11) by adding a promise so that test does not try to connect to a nginx that's not started yet

```
======================================================================
ERROR: test (test.TestJSTestNode)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "parts/slapos-repository/software/jstestnode/test/test.py", line 60, in test
    '%sjio/test/tests.html' % (connection_dict['nginx'], ), allow_redirects=False)
  File "eggs/requests-2.20.0-py2.7.egg/requests/api.py", line 75, in get
    return request('get', url, params=params, **kwargs)
  File "eggs/requests-2.20.0-py2.7.egg/requests/api.py", line 60, in request
    return session.request(method=method, url=url, **kwargs)
  File "eggs/requests-2.20.0-py2.7.egg/requests/sessions.py", line 524, in request
    resp = self.send(prep, **send_kwargs)
  File "eggs/requests-2.20.0-py2.7.egg/requests/sessions.py", line 637, in send
    r = adapter.send(request, **kwargs)
  File "eggs/requests-2.20.0-py2.7.egg/requests/adapters.py", line 516, in send
    raise ConnectionError(e, request=request)
ConnectionError: HTTPConnectionPool(host='::1', port=9443): Max retries exceeded with url: /jio/test/tests.html (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f9b01f4b310>: Failed to establish a new connection: [Errno 111] Connection refused',))

```

See merge request !733