JSON Schema for versioned notifications

Versioned notifications in nova have a well defined schema implemented with
versioned objects. This schema prevents from changing the structure of the
notification without communicating the change to the notification consumers.
This communication is done via the version number embedded into the
notification payload. However this communication is limited and does not
contain information about what is changed in the schema or what is the
structure of a new notification.

Notification consumers can understand the structure of the notification payload
based on notification samples or based on the object model only.
If a notification payload is changed then the object model is updated. However
this information is hard to consume from software and the consumer shall not
depend on the object model of the service directly.

Also the consumer does not have an independent and automatic way to ensure that
the received notification has the expected structure without trying to consume
it.

The versioned notification transformation work [1] is progressing well in nova
and the JSONSchema work progressing well in o.vo [2]. We think that
nova can start providing JSONSchemas for the versioned notifications
to the notification consumers.

The following steps are planned for Ocata as part of this bp:
* Implement the get_schema() method for the nova specific o.vo fields similarly
  to how it was implemented to to the o.vo generic fields during [2].
* Implement a script under ./tools directory in nova that generates the JSONSchema
  of the exising versioned notification payload classes.
* Update the notification devref to contain the schemas as well.

[1] https://blueprints.launchpad.net/nova/+spec/versioned-notification-transformation-ocata
[2] https://review.openstack.org/#/q/branch:master+topic:bp/json-schema-for-versioned-object