Comment 4 for bug 1872755

I did some spelunking and it looks like the trust_id used to be used to add the correct roles to a token requested with an ec2 credential, and that functionality was mistakenly removed in https://review.opendev.org/389365. The consequence is documented in https://bugs.launchpad.net/keystone/+bug/1872735 so I propose we close this as a duplicate of that one.