Remove superfluous role_name arguments
This patch removes the role_name when using openstack_user's :create
action (it gets ignored by the target method).
Note that the spec test would still pass if only the line in
identity_registration.rb (but not the test) were changed, because the
code that actually does grant the role to the resource is executed right
after user creation and before any tests check the resource for the
existence of the role_name attribute. In other words: if the argument
were required in a call but only supplied in another call, the spec
tests would not catch it. Something to watch out for.