Historically Debian cloud images included backports sources by default
(apparently to ensure that new enough cloud-init could be installed to
boot on various clouds). DIB mimicked this behavior and included
backports by default in the debian-minimal element. Since then packaging
for cloud-init has changed and now upstream cloud images no longer
include backports by default. Update debian-minimal to match.
Users of debian-minimal can include backports again by setting the
DIB_APT_SOURCES_CONF variable in their builds. Or they can add the
source as a separate source.list.d entry with custom elements.
As part of this change the debootstrap element docs are also updated to
reflect the change. One thing that was noticed is that the
ubuntu-minimal element uses different variables to manage the apt
sources despite relying on the common framework from the debootstrap
element. The debootstrap docs are updated to include a note about this
difference.
Change-Id: Id9f0cacf7707709829e49006d26d6c98dc8a8ba5
Signed-off-by: Clark Boylan <clark.boylan@gmail.com>
4987b7dbc3