Attribute consistency proposal

There are two semi-related proposals here:

1. Make node 'normal' attributes non-persistent, and implement a new API for node-specific persistent data
2. Access system profiling attributes via a different API than user-controlled attributes.
Also, it's inconsistent with the URL shown on [line 153](https://github.com/opscode/chef-rfc/pull/9/files#diff-3b4340a071fe4ad8c7b60649bdf84d6dR153). I'd say this document should use either the current _or_ the new docs URL, but not both.