clustersettings: support tenants to change some settings

[tbg]

Background

We need to come up with a story around cluster settings for tenants at some point in the future (post phase 2). The story could be that tenants can't change "cluster" (their own logical cluster) settings. Or the story could be that we do allow it at least for some settings and do it via the infra layer (i.e. log into some ui and set the settings there, which propagates them into their logical cluster). Or the story could be that they get to do so via SQL. In either of the last two options, the values would be backed by the tenant's copy of system.settings and retrieved via the mechanism developed for #47150. (cc @ajwerner)

[ajwerner]

I'm calling this done.