Emit service.create notification when new nova service is registered

Nova already emits service.update notification when certain parameters of nova service change due to operator actions (i.e. service disable, service force_done, etc...) However when the data center is expanded and new nova services are added to the system nova does not send notifications. Some external system needs to detect deployment expansion. For example vitrage needs to add the new nova computes to the correlation graph.

Work items:
* Add a new service.create notification that uses the same ServiceStatusPayload [1] that the existing service.update notification uses.
* Make sure that the new service.create notification is emitted when a new nova service is created in the db [2]
* Add unit and functional test coverage
* Add a payload sample to the doc

[1] https://github.com/openstack/nova/blob/5ebff5071dd1f6b542bb8d4ae76aa94ef12d8aa7/nova/notifications/objects/service.py#L33
[2] https://github.com/openstack/nova/blob/master/nova/objects/service.py#L343

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This is a duplicate of https://blueprints.launchpad.net/nova/+spec/service-create-destroy-notification