Add new site config sub-system intended to replace region (#13672)

- New sub-system has "region" and "name" fields.

- `region` subsystem is marked as deprecated, however still works, unless the
new region parameter under `site` is set - in this case, the region subsystem is
ignored. `region` subsystem is hidden from top-level help (i.e. from `mc admin
config set myminio`), but appears when specifically requested (i.e. with `mc
admin config set myminio region`).

- MINIO_REGION, MINIO_REGION_NAME are supported as legacy environment variables for server region.

- Adds MINIO_SITE_REGION as the current environment variable to configure the
server region and MINIO_SITE_NAME for the site name.

cmd/streaming-signature-v4.go

@@ -74,7 +74,7 @@ func calculateSeedSignature(r *http.Request) (cred auth.Credentials, signature s
 	v4Auth := req.Header.Get(xhttp.Authorization)
 
 	// Parse signature version '4' header.
-	signV4Values, errCode := parseSignV4(v4Auth, globalServerRegion, serviceS3)
+	signV4Values, errCode := parseSignV4(v4Auth, globalSite.Region, serviceS3)
 	if errCode != ErrNone {
 		return cred, "", "", time.Time{}, errCode
 	}