fix whitespace issues and de-cuddle elsif/else branches
remove &FAI:: from 'local' functions

There is no need to refer to create_fstab_line(), get_fstab_key() and
find_boot_mnt_point() as &FAI::[...] inside Fstab.pm. Getting rid of
this also helps in moving Fstab.pm into it's own namespace later on.
convert find_boot_mnt_point() to use argument instead of %FAI::configs

old function signature:

  my $boot_mnt_point = find_boot_mnt_point();

new function signature:

  my $boot_mnt_point = find_boot_mnt_point(%FAI::config);

This change does not change the actual functionality. Providing the
configuration via a function argument makes it possible to get rid
of the global variable %FAI::configs later on. As an immediate
benefit it is now obvious that find_boot_mnt_point() uses the data
stored in %FAI::configs to find the boot device.
get rid of %FAI::configs, convert hash reference $config into an actual hash
move update of %FAI::disk_var from create_fstab_line() to generate_fstab()

Updating $FAI::disk_var{ROOT_PARTITION} and $FAI::disk_var{SWAPLIST} in
create_fstab_line() is a bit surprising, because the function name does
not indicate such a thing may happen. The function generate_fstab() already
updates $FAI::disk_var{BOOT_DEVICE} so it's better to consolidate the
modifications to %FAI::disk_var into one place.

This is the patch is the first one in a series to replace the global
variable with a suitable return value.
reformat conditionals to prepare for optimization
refactor Fstab.pm

 - generate disk_vars from scratch instead of extending existing value
   (generate_fstab can now be called multiple times without corrupting
    the swaplist)
 - replace repeated use of hash values with variables
   (avoid accidental mistyping of hash key)
remove global variable %FAI::disk_var

The function signature for generate_fstab() has changed:

old:

  my @fstab = &FAI::generate_fstab(\%FAI::configs);

new

  my ($aref, $href) = &FAI::generate_fstab(\%FAI::configs);
  my @fstab    = @{ $aref_fstab    };
  my %disk_var = %{ $href_disk_var };

'generate_fstab()' now longer uses / modifies the global variables
%FAI::configs and %FAI::disk_var and can be called multiple times
without corrupting the value of $disk_var{'SWAPLIST'}.

All provided testcases still verify successfully. setup-storage has
been adjusted to use the new signature.