Document driver_info external_http_url
Document driver_info external_http_url

This commit adds documentation about driver_info[external_http_url]
Follow-up If6a117a756b7d2a04251792f88c2ee412a040b28

Change-Id: Ia4787c27ed4c53f4ecb911eb0f9d77ea455c25f3

openstack/ironic,openstack/ironic