The RPC transport_url for keystone was being set in the DEFAULT
section, even though keystone doesn't do anything with it. Instead,
keystone leans on the [oslo_messaging_notification] section from
oslo.messaging to register the transport_url option.
This change sets the transport_url in the proper section instead of
using the DEFAULT section.
Change-Id: I11590d0175da7ea310d5529f2d7c0bf8d7fb25b3