refactor FAI::get_current_lvm() to use parse_vgdisplay()
use SetupStorage::LVM::parse_vgdisplay() to create/parse LVM configuration

 - add a method to disable abs_path() during testing
   (make tests independent of locally existing symlinks)
 - create new library SetupStorage::Units, which provides convert_size()
   (convert_size() is intended to completely replace FAI::conver_unit())
 - rename vgdisplay output files used for testing