Remove unused method _get_domain_id_from_auth

The _get_domain_id_from_auth method in
keystone.token.controllers.Auth is not used or tested, so remove it.

Change-Id: Ifb20d419f32cbddf7e3fb126b5b8ecfaa50b582b
Sync db.exception from Oslo

Change-Id: I7ff5e8920a84b1c3cc59278fb6e21b6bf2913733
Closes-Bug: #1273301
Merge "Sync db.exception from Oslo"
Merge "Remove unused method _get_domain_id_from_auth"