jobs/swh-mirror: Update job configuration and fix its execution
Run job in the Jenkins docker agent.
Add support for testing merge requests.
Ensure a docker swarm cluster with required labels is created prior running tests.
Update kafka broker URL.
Nevertheless, credentials for the kafka broker on staging must be updated as the following error is reported (see https://jenkins.softwareheritage.org/job/DFP/job/run-tests/526/consoleFull):
ERROR:swh.objstorage.replayer.replay:Failed operation get_object on 7e3d4d9741e269a4889eb2e3b8361ee1021f1648 after 3 retries; last exception: ReplayError(7e3d4d9741e269a4889eb2e3b8361ee1021f1648, APIError(\'<html>\\r\\n<head><title>403 Forbidden</title></head>\\r\\n<body>\\r\\n<center><h1>403 Forbidden</h1></center>\\r\\n<hr><center>nginx</center>\\r\\n</body>\\r\\n</html>\\r\\n\', <Response [403]>), [\' File "/usr/lib/python3/dist-packages/swh/objstorage/replayer/replay.py", line 183, in get_object\\n obj = objstorage.get(obj_id)\\n\', \' File "/usr/lib/python3/dist-packages/swh/core/api/__init__.py", line 190, in meth_\\n return self._post(meth._endpoint_path, post_data)\\n\', \' File "/usr/lib/python3/dist-packages/swh/core/api/__init__.py", line 285, in _post\\n return self._decode_response(response)\\n\', \' File "/usr/lib/python3/dist-packages/swh/core/api/__init__.py", line 372, in _decode_response\\n self.raise_for_status(response)\\n\', \' File "/usr/lib/python3/dist-packages/swh/core/api/__init__.py", line 362, in raise_for_status\\n raise exception from None\\n\'])\n' (test_graph_replayer.py:101)