storage/create-keyspace: Extract cassandra configuration when possible

It expected a simple cls cassandra so far but it does not reflect the production/staging use case (we deploy storage cassandra with pipeline configuration).

And we currently do that kind of configuration mangling in the chart [1]. With this, we will be able to simplify a lot this code (just run in on the plain storage configuration without having to do some massaging first).

[1] https://gitlab.softwareheritage.org/swh/infra/ci-cd/swh-charts/-/blob/production/swh/templates/utils/backend-utils.yaml?ref_type=heads#L271-349