Drush 6.6.0 Site-Install fails with 'No database to operate on.'
Your --db-url is not in the correct format.  Use this instead:

--db-url=mysql://USER@data.rds.awsdomain.com/dbname

Replace USER with the account used to access the database.  If this is the same user that you have specified with --db-su, then the --db-su option is not necessary.  Drush will not go the other way (fill in the "ordinary" user from the privileged user specified via --db-su), though.