Refactor nbi-notifications

Refactor nbi-notification to rationalize the code, according to TRNSPRTPCE-638. This means defining a baseline for notification provided through the kafka broker (Topics, message+ specific content that will depends on the notification type/topic). This shall include the management of notification on path-computation-result according to the need expressed by UTD through their contribution (code used during the OFC demo to provide information on the path to be established).