Switch to decorators.idempotent_id
Switch to decorators.idempotent_id

test.idempotent_id is being kept temporary to migrate
to new lib interface. Now idempotent_id is available as Tempest stable
interface decorators and all plugins tests using the old decorator
should be switched to new interface.

In future, Once all plugins are switched to new decorator Tempest
will remove the test.idempotent_id

This commit correct the example tests in doc.
Related-Bug: 1616913

Change-Id: I4c8eb461a04072403d5bf0e03cefae8df03999f0

openstack/designate,openstack/designate,openstack/designate