Multi-step workflow support

Supporting multi-step workflows (e.g. select image -> create keypair -> launch) is going to be a key part of Horizon's usability. We need to create a reusable component with a simple, straightforward API that abstracts all the complexity involved in this and makes it not only easy to use from the end-user's standpoint, but easy to build for developers.

This blueprint would likely be a significant milestone for the Folsom release cycle, and needs significant design work before any development is begun.