refactor / code cleanup

 - FAI::get_current_disks() reimplemented as
   SetupStorage::Devices::convert_diskconfig()
 - FAI::find_all_phys_devs() reimplemented as
   SetupStorage::Devices::find_required_devices()

old  &FAI::get_current_disks;
new  my ($href, $aref) = convert_diskconfig(\@disks, \%cur_dev_children, \%configs, $dryrun);
new  my %c_config = %{ $href };
new  my @commands = @{ $aref };

old  @devices = @{ &FAI::find_all_phys_devs() };
new  @devices = find_required_devices(%configs);