Refactor fstab.pm 'generate_fstab()'

Please review this patchset to get rid of the global variable %FAI::disk_var and the implicit usage of %FAI::configs in find_boot_mnt_point().

Patches can be applied one after another, alls patches verify successfully against the unit test. The final patch requires a modification of the setup-storage script. (included in the patch) The unit test results were generated by an unmodified setup-storage 1.6.

 (This is one of the steps necessary to turn Fstab.pm into a proper module with it's own namespace.)