Architecture values files to reuse dict information across namespaces

@olasd @vsellier What about this organization?

That way:

• the /swh/values/{environment}/default.yaml contains the common/default values of the environment without having to duplicate it (as we do currently and it's quite tedious).
• the /swh/values/{environment}/{namespace}.yaml holds the specifics (diverging) values of the namespace
• the /swh/values/{environment}/overrides/{namespace}-with-some-more-specificities.yaml (e.g. swh-cassandra-next-version) can still be used as a composition of values

In [1] you can find the corresponding adaptations (without any diff for staging and production namespaces).

[1] swh-charts!173 (merged)