PaaS Event Format for Ceilometer Collection

Provides a payload specification to standadardize the format of PaaS
integrations so that Ceilometer can consume metering data from any
conforming application without custom integration.

DocImpact

Change-Id: Icbb141b7aaf402d18d0a7e9e08a7b816d9833e6c
Merge "PaaS Event Format for Ceilometer Collection"