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.

config/config.json

@@ -124,7 +124,8 @@
         "AmazonS3Bucket": "",
         "AmazonS3Region": "us-east-1",
         "AmazonS3Endpoint": "s3.amazonaws.com",
-        "AmazonS3SSL": true
+        "AmazonS3SSL": true,
+        "AmazonS3SignV2": false
     },
     "EmailSettings": {
         "EnableSignUpWithEmail": true,