Updated pull request for fstab.pm

This request extends my previous pull request <https://github.com/faiproject/fai/pull/17>

"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()."

This pull requests adds 2 new patches
 - patch 1: generate_fstab() now returns the actual content of disk_var.sh instead of just a hash
 - patch 2: update unit tests to reflect the change

The expected output for @disk_var has been generated by using an unmodified
setup-storage 1.4. There is a small difference introduced by the patches: Previously
the order of the variables in disk_var.sh was random. After applying this fix the order
of variables is alphabetically sorted. (This was done to prevent the tests from failing
randomly.)

My branch with the changes is located at
https://github.com/ThomasNeumann/setup-storage/tree/refactor_Fstab.pm+1