image-guide: update information about SLES12 images
image-guide: update information about SLES12 images

* subscribtion is not required to download the image, an existing account is sufficient
* images are provided on download.suse.com

Change-Id: I5d607fed08a5d0a4efff29b2e7b4457d972e94aa

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals