Skip stub grub.cfg files (e.g. used on Debian OS family).

Replacing these may break OS update mechanisms which only update
the main file.

lib/puppetx/augeasproviders_grub/util.rb

@@ -150,12 +150,21 @@ def self.grub2_cfg_paths
         ]
 
         valid_paths = paths.map do |path|
-          begin
+          real_path = nil
+
+          if File.readable?(path) && !File.directory?(path)
             real_path = File.realpath(path)
-            real_path if (File.readable?(real_path) && !File.directory?(real_path))
-          rescue Errno::ENOENT
-            nil
+
+            # Exclude stub files which include main config (e.g. Debian OS family)
+            File.foreach(real_path) do |line|
+              if line.match(/^configfile\s/)
+                real_path = nil
+                break
+              end
+            end
           end
+
+          real_path
         end.compact.uniq
 
         fail(%{No grub configuration found at '#{paths.join("', '")}'}) if valid_paths.empty?