Better validation of all config file fields (#6090)

Add Validate() to serverConfig to call it at server
startup and in Admin SetConfig handler to minimize
errors scenario after server restart.

pkg/event/target/elasticsearch.go

@@ -18,8 +18,10 @@ package target
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/url"
+	"strings"
 	"time"
 
 	"github.com/minio/minio/pkg/event"
@@ -36,6 +38,26 @@ type ElasticsearchArgs struct {
 	Index  string   `json:"index"`
 }
 
+// Validate ElasticsearchArgs fields
+func (a ElasticsearchArgs) Validate() error {
+	if !a.Enable {
+		return nil
+	}
+	if a.URL.IsEmpty() {
+		return errors.New("empty URL")
+	}
+	if a.Format != "" {
+		f := strings.ToLower(a.Format)
+		if f != event.NamespaceFormat && f != event.AccessFormat {
+			return errors.New("format value unrecognized")
+		}
+	}
+	if a.Index == "" {
+		return errors.New("empty index value")
+	}
+	return nil
+}
+
 // ElasticsearchTarget - Elasticsearch target.
 type ElasticsearchTarget struct {
 	id     event.TargetID