feat(stacks): update wordpress template

stack_templates.json

@@ -33,6 +33,13 @@
     "repository": {
       "url": "https://github.com/portainer/templates",
       "stackfile": "stacks/wordpress/docker-stack.yml"
+    },
+    "env": [
+      {
+        "name": "MYSQL_ROOT_PASSWORD",
+        "label": "Database root password",
+        "description": "Password used by the MySQL root user."
       }
+    ]
   }
 ]