After the 3 machines was rebooted, I've found that `/var/lib/nova/intances` (Ceph + Nova live migration) wasn't mounted automatically, and `openstack` root partition (`/`) was getting full (https://dashboard.openstack.sj.ifsc.edu.br/snmp). So, this commit just fix it.
At the first sight, I imagined a 'superadmin' user, above 'admin'. However, many files, specially policies (`policy.json`) was to be modified, because 'admin' role supersedes tenant roles. So, instead of a complex solution, a better on (as it seams to me): create a simple role, `horizon_dashbord` (based on `heat_stack_user`) and modify Dashboard panels permissions.