Hi,
According to recent discussion with Marion and David I changed authentication model for instances. I added new module Deltacloud::Authentication with two classes 'Password' and 'Key'.
Also I added a 'password' authentication to Rackspace, but it's unfortunatelly returned only after 'create' operation.
All patches are tested on GoGrid/EC2, I'll be glad if someone will take Rackspace testing.
-- Michal