libzmqutil: log successful auth at DEBUG level

Problem: successful authentication is logged at INFO level,
but these message are routine and usually followed by an "accepting
connection" message when a peer connects.

Drop the log level to DEBUG.

Author: garlick

src/common/libzmqutil/zap.c

@@ -123,7 +123,7 @@ static void zap_cb (flux_reactor_t *r,
             status_text = "OK";
             user_id = pubkey;
             name = zcert_meta (cert, "name");
-            log_level = LOG_INFO;
+            log_level = LOG_DEBUG;
         }
         if (!name)
             name = "unknown";