Rackspace - support setting admin pass during server creation

This PR fixes the Rackspace provider so it lets you set the admin password while creating a server.  The Fog documentation said the password was a read-only attribute only available in the response, but the service documentation says it is a read-write attribute.

This helps clear the way for Windows servers, because they don't generally support SSH.  Since you can't rely on uploading a public key, setting the password to a known value is useful.  If it is randomly generated, then projects like vagrant-rackspace would need to capture and persist the password.  It's easier to just let the vagrant user select the password and provide it to fog.

Despite being a relatively small diff, it did cause me a lot of confusion, as there's a few special things happening with the password (i.e. the `adminPass` and `change_admin_password` methods).