Add mapping for ceilometer agent auth_region

I5fc5ae1b41 added several missing region data mappings.
However, one further mapping is also needed: we don't currently
have a mapping for the region setting in the ceilometer agent.
This patch adds a mapping for the
ceilometer::agent::auth::auth_region parameter.

Change-Id: I66e890737bbbd00a296e0bfa861ca8992b472c17
Closes-Bug: #1308366