Refactor validation of presence of extracted config-files

The actual validation that all expected files exist is moved from
SystemDescriptionStore to SystemDescription. That way the store class
doesn't have to have knowledge about scope internals like the deleted
flag.

Eventually the validation will be moved to the specialized scope
classes, but for now I think it's cleaner to have it in the
SystemDescription rather than the store class.
Also check for presence of extracted changed managed files
Also validate the presence of extracted unmanaged files

Unmanaged files aren't just copied over like config or changed managed
files, but are stored in tarballs instead. For performance reasons we do
not inspect the tarballs for the specific files during validation,
though, but only make sure that all tarballs are around.
Mention file presence checks in "machinery validate" man page
Also look for a files tarball if there are only extracted link(s)
Merge pull request #186 from SUSE/review_140905_validate_extracted_unmanaged_files

[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_140905_validate_extracted_unmanaged_files'