Reference the software stories image in the top-level values

So it can be updated through the ci pipeline.

This also unifies the declaration with other charts.

tests:

• Unit tests are happy
• minikube is happy too (& contrary to the swh charts where services expectedly fail there, this one is actually fully running within minikube, pretty neat)

I'll go and adapt the argocd application so the 'host' parameter override happening there is removed from the declaration (as this mr obsoletes it).