store: Expose S3 region attribute

Minio is able to autodetect the region for cloud providers like AWS but the logic fails with Scaleway Object Storage solution.

Related issue on Minio: minio/mc#2570

docs/storage.md

@@ -49,6 +49,7 @@ type: S3
 config:
   bucket: ""
   endpoint: ""
+  region: ""
   access_key: ""
   insecure: false
   signature_version2: false
@@ -62,7 +63,7 @@ config:
     enable: false
 ```
 
-AWS region to endpoint mapping can be found in this [link](https://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region)
+The variable `region` is optional. AWS region to endpoint mapping can be found in this [link](https://docs.aws.amazon.com/general/latest/gr/rande.html#s3_region).
 
 Make sure you use a correct signature version.
 Currently AWS require signature v4, so it needs `signature-version2: false`, otherwise, you will get Access Denied error, but several other S3 compatible use `signature-version2: true`

pkg/objstore/s3/s3.go

@@ -37,6 +37,7 @@ const DirDelim = "/"
 type Config struct {
 	Bucket          string            `yaml:"bucket"`
 	Endpoint        string            `yaml:"endpoint"`
+	Region        	string            `yaml:"region"`
 	AccessKey       string            `yaml:"access_key"`
 	Insecure        bool              `yaml:"insecure"`
 	SignatureV2     bool              `yaml:"signature_version2"`
@@ -122,7 +123,7 @@ func NewBucketWithConfig(logger log.Logger, config Config, component string) (*B
 		}
 	}
 
-	client, err := minio.NewWithCredentials(config.Endpoint, credentials.NewChainCredentials(chain), !config.Insecure, "")
+	client, err := minio.NewWithCredentials(config.Endpoint, credentials.NewChainCredentials(chain), !config.Insecure, config.Region)
 	if err != nil {
 		return nil, errors.Wrap(err, "initialize s3 client")
 	}