Expose multiple APIs from an rpc endpoint

It has become increasingly clear in nova that the ability to expose more than one API over rpc from a single service would be useful. Specifically, we have had a couple of cases now where we would like to add methods that apply to *all* services. There is no good way to do that right now. This blueprint is for adding the ability to expose multiple APIs, that exist within their own namespace, and are versioned independently of each other.