Implement nova instances plugin

One of the 'must haves' for liberty is indexing nova instances (allowing the demo we showed at the summit). The existing glance plugins are good templates to use. Both indexing (from scratch and from notifications) and searching should be supported. The ultimate goal is to allow Horizon to consume Searchlight for querying nova instances.

Summit prototype code in https://github.com/sjmc7/glance/tree/nova-support

The summit prototype pulled in (for instance) glance image information which meant additional calls out at index-time. From a search perspective this makes sense, but from an index speed and complexity perspective denormalization adds difficulty. This is a good case for experimentation, but it's also a case to see if we can get that information included in notifications.

Tests should be included for the same areas that the glance plugins are tested:
 - RBAC
 - any output filtering that's required
 - any serialization that's required both from notifications and from initial indexing

There'll be a dependency on python-novaclient introduced for initial indexing.

We should make sure all plugins are documented to a) make them visible and b) explain what and how they're doing.

We should be able to show the original data and augment it here and there for searchability, etc, but to always provide raw data.

We also need to ensure that we provide source field exclusions for some fields if the user is not an admin. For example, the host, hypervisor, etc.