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/api-headers.go

@@ -52,7 +52,7 @@ func setCommonHeaders(w http.ResponseWriter) {
 
 	// Set `x-amz-bucket-region` only if region is set on the server
 	// by default minio uses an empty region.
-	if region := globalServerRegion; region != "" {
+	if region := globalSite.Region; region != "" {
 		w.Header().Set(xhttp.AmzBucketRegion, region)
 	}
 	w.Header().Set(xhttp.AcceptRanges, "bytes")