blueprint versioning

Blueprints change rather often - there are two major ways they change that we should care about:
- the resources the blueprint produces change
- the blueprint class itself somehow changes, like if we add/remove arguments in **init**

In case 1 it'd be good to make sure that if there is a potentially dangerous change that would require the destruction of resources that we let people know that.

In case 2 - well, we need to make sure that the blueprints match their expected behavior in stacker.

Not sure how to handle this.