[RFC] Define Flux tenancy models

[stefanprodan]

The main goal of this RFC is to define the Kubernetes tenancy models supported by Flux.

This PR attempts to document the status quo, and should provide clarity of what multi-tenancy capabilities Flux has. It also functions as a base for rewriting the loose proposal in #582 into well scoped RFCs.

[squaremo]

It's not uncommon to have a "memorandum" RFC which describes the status quo, rather than proposing a new design. It seems like a needless indirection to use an RFC to propose new documentation, giving the content verbatim, though. I would expect either

• an RFC which will stand itself as the description of the status quo at some point (and, e.g., documentation can point at it, or paraphrase it while still relevant); or,
• a PR with the new documentation.

Given the goal of building up to new designs, I think the first is the appropriate form here (and would require only a little adaptation).