The role_assignment module always looks up the user, group and project
so to support cross-domain assignments we should add extra parameters
like OSC to look them up from the correct domains. Switch to using
the service proxy interface to grant or revoke the roles as well.
Partial-Bug: #2052448
Partial-Bug: #2047151
Partial-Bug: #2097203
Change-Id: Id023cb9e7017c749bc39bba2091921154a413723