Introduce instance notifications for inventory-related events

There seem to be a number of notifications defined in OpenFlow models which bear a yang-ext:context-reference to the underlying OFP node.

These notifications are better expressed through instance (or nested) notifications introduced YANG 1.1.

Add equivalent notification definitions in the corresponding data nodes and emit both notifications when the notification event occurs. Also migrate internal users to use instance notifications and deprecate the old notifications.

This will create an explicit migration step for users, where they can use both old and new notifications.