SPEC-3408: fix packer build

SPEC-3408: fix packer build

The current packer templates use 1604.4 which does not exists anymore,
so change it to 1604.6.

Furthermore, jessie-backports does not exists anymore and fail the whole
build.
Since apt-get-update fail (due to jessie-backports), the subsequent
"&& apt-get install" is never executed
Jessie-backports seems needed only for lava-tool which was replaced by
lavacli, so no regrets, remove it.

Change-Id: Iae1333a52a2dc20e9559462b4acf9a3b71696a08
Signed-off-by: Corentin LABBE <clabbe@baylibre.com>