Add missing file store wrapper to SystemDescription
Merge pull request #198 from SUSE/add_missing_filestore_wrapper

Add missing file store wrapper to SystemDescription
Simplify missing_files method

By moving the missing_files method to SystemDescription calling it
and its implementation become simpler
Extract methods to get missing files
Extract method to format missing file errors
Eliminate intermediate map of missing files
Don't return unused value
Deduplicate scope validation code

This requires introduction of a "missing_files_for_scope" method,
which switches a method based on the scope. This clearly should
rather be solved by polymorphism in the scope classes, but this is
a step for later.
Merge branch 'refactor_file_validation'

Pull request #201
Refactor list_file_store_content

Rename list_file_store_content to list_description_content and move
from lib/system_description_store.rb to lib/system_description.rb due
to the refactor in pull-request #201.
Add additional_files method

This method returns a list of files that have no meta informations or
invalid meta informations e.g. files in deleted state.
Implement test cases for the validation of existence meta data
Implement the check for files that have no meta data

Use the additional_files method to get a list of files that have no
meta data.
Refactor the error output method to get the scope title only once.