creds: Secretkey should be generated upto 40 characters in length. (#4471)

Current code allowed it wrongly to generate secret key upto 100
we should only use 100 as a value to validate but for generating
it should be 40.

Fixes #4470

cmd/gateway-main.go

@@ -19,11 +19,12 @@ package cmd
 import (
 	"errors"
 	"fmt"
-	"github.com/gorilla/mux"
-	"github.com/minio/cli"
 	"net/url"
 	"os"
 	"strings"
+
+	"github.com/gorilla/mux"
+	"github.com/minio/cli"
 )
 
 var gatewayTemplate = `NAME: