stages/yum.config: add an option to configure langpacks plugin

The new stage enables users to configure the langpacks plugin of YUM.
Currently, only locales option is supported.

test/data/stages/yum.config/b.json

@@ -498,6 +498,13 @@
         "options": {
           "config": {
             "http_caching": "packages"
+          },
+          "plugins": {
+            "langpacks": {
+              "locales": [
+                "en_US.UTF-8"
+              ]
+            }
           }
         }
       }

test/data/stages/yum.config/b.mpp.json

@@ -34,6 +34,13 @@
         "options": {
           "config": {
             "http_caching": "packages"
+          },
+          "plugins": {
+            "langpacks": {
+              "locales": [
+                "en_US.UTF-8"
+              ]
+            }
           }
         }
       }

test/data/stages/yum.config/diff.json

@@ -1,6 +1,9 @@
 {
   "added_files": [
-    "/etc/yum.conf"
+    "/etc/yum.conf",
+    "/etc/yum",
+    "/etc/yum/pluginconf.d",
+    "/etc/yum/pluginconf.d/langpacks.conf"
   ],
   "deleted_files": [],
   "differences": {}