swh-journal: Add tests to the journal publisher using kafka instance

Testing the publisher with a real kafka instance through the pytest-kafka dependency.

This also:

• documents with a script how and what to install in regards to the kafka needed
• extracts the configuration checks done on the publisher via the cli and push those checks in the publisher directly.
• those checks are now tested (they helped in the debugging process of those tests)
• update the readme

Related T1276 Depends on D1280 Depends on D1282

Test Plan

tox

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Migrated from D1239 (view on Phabricator)