Denormalize nova image/flavor

The early prototype for nova servers denormalized image and flavor names but after some discussion this was dropped because it meant even more API requests on notifications (one to glance, one to nova).

If one day the notifications contain all this, then great, but until then, another (faster) option is using the information stored in searchlight to do the denormalization; requesting a flavor or image from Elasticsearch is an order of magnitude faster than from nova or glance (presuming the data's indexed - if it's not, there's not much that can be done although it'd be easy to order the initial data import).

This complicates nova server serialization somewhat but adds a lot of value. It would be good to gauge interest.

Resolve with: https://blueprints.launchpad.net/searchlight/+spec/disoverable-resource-type-mapping