Support AWS Signature V2 for Mattermost for S3 storage. (#6462)

Certain S3 compatible servers only use Legacy Signature (AWS
Signature V2), current code only supports signature v4.

This PR adds facility to click a button on the UI to enable
legacy signature with S3 compatible servers.

api/user_test.go

@@ -20,8 +20,6 @@ import (
 	"github.com/mattermost/platform/model"
 	"github.com/mattermost/platform/store"
 	"github.com/mattermost/platform/utils"
-
-	s3 "github.com/minio/minio-go"
 )
 
 func TestCreateUser(t *testing.T) {
@@ -696,7 +694,8 @@ func TestUserCreateImage(t *testing.T) {
 		accessKey := utils.Cfg.FileSettings.AmazonS3AccessKeyId
 		secretKey := utils.Cfg.FileSettings.AmazonS3SecretAccessKey
 		secure := *utils.Cfg.FileSettings.AmazonS3SSL
-		s3Clnt, err := s3.New(endpoint, accessKey, secretKey, secure)
+		signV2 := *utils.Cfg.FileSettings.AmazonS3SignV2
+		s3Clnt, err := s3New(endpoint, accessKey, secretKey, secure, signV2)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -800,7 +799,8 @@ func TestUserUploadProfileImage(t *testing.T) {
 			accessKey := utils.Cfg.FileSettings.AmazonS3AccessKeyId
 			secretKey := utils.Cfg.FileSettings.AmazonS3SecretAccessKey
 			secure := *utils.Cfg.FileSettings.AmazonS3SSL
-			s3Clnt, err := s3.New(endpoint, accessKey, secretKey, secure)
+			signV2 := *utils.Cfg.FileSettings.AmazonS3SignV2
+			s3Clnt, err := s3New(endpoint, accessKey, secretKey, secure, signV2)
 			if err != nil {
 				t.Fatal(err)
 			}