Generalize loadConfig method to avoid reading from disk (#5819)

As we move to multiple config backends like local disk and etcd,
config file should not be read from the disk, instead the quick
package should load and verify for duplicate entries.

cmd/admin-handlers.go

@@ -33,6 +33,7 @@ import (
 	"github.com/minio/minio/pkg/auth"
 	"github.com/minio/minio/pkg/handlers"
 	"github.com/minio/minio/pkg/madmin"
+	"github.com/minio/minio/pkg/quick"
 )
 
 const (
@@ -693,7 +694,7 @@ func (a adminAPIHandlers) SetConfigHandler(w http.ResponseWriter, r *http.Reques
 
 	// Validate JSON provided in the request body: check the
 	// client has not sent JSON objects with duplicate keys.
-	if err = checkDupJSONKeys(string(configBytes)); err != nil {
+	if err = quick.CheckDuplicateKeys(string(configBytes)); err != nil {
 		logger.LogIf(ctx, err)
 		writeErrorResponse(w, ErrAdminConfigBadJSON, r.URL)
 		return