Add RBAC support to Barbican, compatible with Keystone and Repose

Add RBAC support to Barbican compatible with Keystone and Repose integrations. The most promising option seems to be using 'eom' (https://github.com/racker/eom/blob/master/etc/rbac.json-sample) as REST-friendly RBAC middleware that does not require modifying the application code behind it. Other options, such as the policy.json approach taken by Glance and Ceilometer require instrumenting application code to support RBAC.