Tighten models

Currently, the data model of Ceilometer has no limitation in theory: for example, there's nothing stating that a resource_id is always linked to only one project_id. While a lot of assumptions are done in various place around that, the schema don't always implement these.

We need to describe a tighter model with better defined relationship in Ceilometer samples and events to build a better APIv3 and a more optimized models, so it can have better performance in the end.