Make swh-journal node run on swh-kafka node

Let other unit tests run as usual on the default swh-tox image.

Related T1276 Related swh/devel/swh-journal!242 (closed) Depends on swh-jenkins-dockerfiles!42 (closed)

Test Plan

$ pip install jenkins-job-builder
$ tox

At the moment, that does not work The plan is something like this:

$ tox > before
$ grep assignedNode before | grep swh-kafka
$ arc patch <this>
$ tox > now
$ grep assignedNode now | grep swh-kafka
<assignedNode>swh-kafka</assignedNode>

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Migrated from D1282 (view on Phabricator)