Fix a few issues with the grub hook

Fix a few issues with the grub hook

This patch:

- updates the comments (no need for explicit EXTRA_PKGS)
- export the proxy if the instance-debootstrap environment defines it
- sets the debconf frontend to non-interactive (I've seen it go into a
  loop otherwise)

I still can't create a grub install, because grub-probe doesn't want
to use LVM names nicely, but that's another issue.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>