Merge lp://staging/~tribaal/livecd-rootfs/xenial-proposed-minimized-round-two into lp://staging/~ubuntu-core-dev/livecd-rootfs/xenial-proposed

* Remove apt, debconf, dpkg cruft files from /var/cache and /var/lib in
  all our livefses.
* Pass --cache false to lb config; otherwise we copy around caches of
  .debs that are never used properly, and which prevent us from emptying
  /var/cache/apt in images.
* When building minimized cloud images, remove various packages that we
  don't want installed by default. Some are tools that aren't needed for
  non-interactive use; some are libraries whose reverse-dependencies
  will have already been removed; and one, open-vm-tools, should only be
  included in images that are targeted to VMWare (which is not the case
  for any of the current minimal images), rather than being included
  directly in the cloud-image seed.

(expanded changelog)