[sonic-cfggen] Load JSON files before minigraph file (#4202)

If sonic-cfggen is passed the -m argument (to load the minigraph file) along with one or more -j <json_file> arguments, load the JSON files before loading the minigraph file.

This ensures that the init_cfg.json file is loaded before the minigraph, therefore the minigraph can override any default configuration options specified in init_cfg.json. Currently, the behavior is reversed.

Note: This is not an issue if loading loading multiple JSON files, because sonic-cfggen loads them in the left-to-right order they were specified on the command line, therefore providing flexibility for loading JSON files in a specific order. As long as init_cfg.json is specified before config_db.json, the values specified in config_db.json will take precedence.

src/sonic-config-engine/sonic-cfggen

@@ -187,6 +187,10 @@ def main():
             sys.exit(1)
         deep_update(data, {'PORT': ports})
 
+    for json_file in args.json:
+        with open(json_file, 'r') as stream:
+            deep_update(data, FormatConverter.to_deserialized(json.load(stream)))
+
     if args.minigraph != None:
         minigraph = args.minigraph
         if platform:
@@ -205,10 +209,6 @@ def main():
             additional_data = yaml.load(stream)      
             deep_update(data, FormatConverter.to_deserialized(additional_data))
 
-    for json_file in args.json:
-        with open(json_file, 'r') as stream:
-            deep_update(data, FormatConverter.to_deserialized(json.load(stream)))
-
     if args.additional_data != None:
         deep_update(data, json.loads(args.additional_data))