Swift/ideas/acl-domains

Problem
Swift ACLs usually apply to username:project tuples. If we use domains, it's possible that there are two or more usernames which are equal on different domains, to identify different people. For this reason, in order to avoid security problems, it's recommended to use user ids instead of usernames. This is not very user friendly.

Proposed solution
Add the possiblity of specifying user and project domains in ACLs, in order to allow again the creation of ACLs with human-readable usernames.

Contact: mpasserini on IRC