Fix to "create user" operation

The "create user" operation is different for Rackspace and OpenStack APIs: Rackspace requires a [`username`](http://docs.rackspace.com/auth/api/v2.0/auth-client-devguide/content/POST_addUser__v2.0_users_User_Calls.html) attribute, OpenStack requires a [`name`](http://developer.openstack.org/api-ref-identity-v2.html#admin-users) attribute. This PR differentiates between clients to make that happen. Should fix #410