Document the standard for PaaS service notifications

This spec defines a minimum data set and optional fields to be
included in the notification (header+payload) produced by PaaS
services.

Implements blueprint paas-event-for-ceilometer

Change-Id: I44218e7f8e67bbb459bd7fa2eda2c7ab06fffef9
Merge "Document the standard for PaaS service notifications"