ceilometer_controller: Add missing ceilometer::agent::auth class

Without this patch, class ceilometer::agent::central was installed on
the ceilometer controller but without the necessary authentication
settings to talk to other openstack services.

Adding the ceilometer::agent::auth class on the controller node fixes
this issue.

Change-Id: I7f1ead998f3f6c9492f5076160b52156ef689b63