Autoinstallers for Debian Buster and Ubuntu Bionic

Autoinstallers for Debian Buster and Ubuntu Bionic
    Buster is identical to Squeeze.
    Bionic requires significant reworking from the Xenial days:
    - The atomic partman-auto recipe ignores the boot partition
      when using LVM, and our pygrub can't read from LVM.  Switch
      to using an expert recipe that matches the atomic one in
      Debian Buster, but with Ubuntu's preferred sizes and filesystems.
    - The preseed value to avoid installing Grub2 instead of old Grub
      is no longer honored, and we can't handle it the way we do in
      Debian because grub-legacy doesn't exist in Ubuntu.  Instead,
      let it install Grub2 and then we'll install and use pv-grub-menu,
      which maintains a menu.lst from the parsed contents of grub.cfg.
    - pv-grub-menu contains a bug - it hardcodes "/boot" as the directory
      pygrub will expect to find kernels and initrds in; this is wrong
      when the system's /boot is on its own partition, and update-menu-lst
      doesn't provide configurable options either in its invocation, a
      parsed pre-existing menu.lst (where the root device and default
      kernel options can be persistently set in comments), or a config
      file.  However, it does source /etc/default/grub after it has set
      this hardcoded variable to "/boot".  We abuse /etc/default/grub
      to empty that variable back out.