Modify existing payload for loadbalancer.delete.start event which is raised when a LBaaS vip is deleted

Existing context for vip.delete.start only contains loadbalancer id as the payload.
This will not help if we want to automate few things from designate perspective like deleting records associated with the loadbalancer.

The current payload:
"payload":{
      "loadbalancer_id":"8c5c5886-2d5c-4826-8632-1678e1217a72"
   }