The canonical location for the artifacts going on is
tarballs.openstack.org/sahara-extra/, so fix the link to use that
and also use https.
Moreover, since the last tarball required for building images
is available on tarballs.openstack.org, remove the last references
to sahara-files for artifacts and documentation
(the new location was used already for a while in few places).
there are still few references to sahara-files,
but they are all about CentOS6 which is no more supported
by diskimage-builder, and should be removed separately.
Change-Id: Iab5a4d50a0abc6ab278837b6a9efd5e30f31c44a
8cdff5d71f