CLOUDSTACK-7143: a better fix for kernel upgrade disk space problems

Increase / and /boot a little bit, to the point that there is enough
space on them to do a kernel upgrade. With this I think we can
decrease total disk space usage back down to 2.5GB.

Docs for the preseed config found at
  http://anonscm.debian.org/gitweb/?p=d-i/debian-installer.git;a=blob_plain;f=doc/devel/partman-auto-recipe.txt;hb=HEAD
CLOUDSTACK-7143: fix a missing apt-get update

In 8e2d06153b3d5ec1540fac1c8fbc97b5d2b58a8e I mistakenly/accidentally
a apt-get update.

As
    https://wiki.debian.org/Multiarch/HOWTO
explains, apt-get update is needed after adding a new architecture.