rest: set center in /repository/add

Set it to 1, ogCP and ogCLI never use this, and this REST API is never
used by the old web console.

src/rest.c

@@ -5400,7 +5400,9 @@ static int og_cmd_post_repository_add(json_t *element,
 				      struct og_msg_params *params,
 				      char *buffer_reply)
 {
-	struct og_repository repo = {};
+	struct og_repository repo = {
+		.center = 1,		/* never used, set it to 1. */
+	};
 	const char *key, *msglog;
 	struct in_addr addr;
 	struct og_dbi *dbi;