ProfileFile implementation

- follow up to driver data migration into new XML data structure
 + implement KIWIProfileFile class to represent data in the .profile
    file
    * this class allows us to gradually move data that is processed
      in the XML changeset and ends up in .profile without duplicating
      code in other places
  + remove the getImageProvileEnvironment method from XML
    * it was just a cover for a _legacy call that we are trying to
      eliminate
    * no longer needed with the new ProfileFile implementation
- remove attempt to write kiwi_luks to the .profile file
  + kiwi_luks is not used, no longer needed
- add diffutils as build requires
  + diff needed by new unit tests
- unit tests for new code