Fix Gateway startup sequence to populate etcd (if set) with bucket info (#7686)

cmd/gateway-main.go

@@ -221,6 +221,11 @@ func StartGateway(ctx *cli.Context, gw Gateway) {
 		logger.FatalIf(err, "Unable to initialize gateway backend")
 	}
 
+	// Populate existing buckets to the etcd backend
+	if globalDNSConfig != nil {
+		initFederatorBackend(newObject)
+	}
+
 	if enableConfigOps {
 		// Create a new config system.
 		globalConfigSys = NewConfigSys()