Add ceilometer::agent::auth.

Adds a new agent::auth manifest to hold common auth
parameters used by the Ceilometer compute and central agents.

This fixes a deployment issue where you would get duplicate
auth parameter errors when trying to configure the compute
and central agent on the same machine.

Fixes Bug #1220810.

Change-Id: I167c6c570bdebb4be2ef99cd0b9cf81f37856d9f