Comment 6 for bug 1183519

Your propasl is only healing simptome. Main issue is ghost-instance-components. So if you want to get merged, i'm suggesting you to raise exception without any notes in database. Security group wouldn't be created it quota exceeded, and it nova-related stuff, so perform catch there, but not here, it's really segnificant.
So, when i finish implementing complete rollback all security group/role creating will be droped from trove-api and moved to taskmanager, because it is architecturaly should be there. If you take a look at ay API-components you will see, that they are only creating rpc calls and only. Because of immaturity, trove has alot of issues, and this is one of them, and you making it more complicated.