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/bucket-handlers.go

@@ -211,7 +211,7 @@ func (api objectAPIHandlers) GetBucketLocationHandler(w http.ResponseWriter, r *
 	// Generate response.
 	encodedSuccessResponse := encodeResponse(LocationResponse{})
 	// Get current region.
-	region := globalServerRegion
+	region := globalSite.Region
 	if region != globalMinioDefaultRegion {
 		encodedSuccessResponse = encodeResponse(LocationResponse{
 			Location: region,