Add method to get ScopeFileStore

Add method to get a ScopeFileStore object for a given scope from
the SystemDescription. This can then be used to operate on the
directory represented by this object.

The code accessing the file store can then be refactored to use
this object instead of the old methods in SystemDescription and
SystemDescriptionStore.
Move Autoyast to ScopeFileStore
Move Html to ScopeFileStore
Move KiwiConfig to ScopeFileStore
Move SystemDescription to ScopeFileStore

This patch moves the usages of the file store methods in
SystemDescription which are not going to be extracted to the
ScopeFileStore class itself to use ScopeFileStore.

The *_file_store methods will go away (with the exception of
scope_file_store, which is SystemDescription's handle to the scope
file store)
Move ChangedManagedFilesInspector to ScopeFileStore
Move Analyze task to ScopeFileStore
Move ConfigFilesInspector to ScopeFileStore
Move UnmanagedFilesInspector to ScopeFileStore
Move description factory to ScopeFileStore
Move KiwiConfig spec to ScopeFileStore
Remove obsolete stub
Move UnamangedFilesInspector spec to ScopeFileStore
Move ConfigFilesRenderer to ScopeFileStore
Fix style issue
Merge branch 'move_to_scope_file_store'

Pull request #491