openstack_releases fix for ceilometer

remove 'xena' from the list of openstack releases that need ceilometer::agent::auth to run instead of ceilometer::agent::service_credentials. I found that with the latest puppet module the following error was thown with 'xena' in the list, but when removed it ran fine:

Error: Could not run: Could not find resource 'Class[Ceilometer::Agent::Auth]' for relationship on 'Exec[ceilometer-upgrade]'