Update ubuntu preseed to include disk variables #1195824

https://bugs.launchpad.net/bugs/1195824   

 Update ubuntu preseed to include disk variables

    expert_disk = true
      false uses atomic "all in /" disk config
    enable_var = true
      false doesn't create separate /var
    var_part_size = size-in-Kb
    root_part_size = size-in-Kb
    enable_hack = true
      false doesn't create hack, so last partition takes all free space