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-notification-handlers.go

@@ -72,8 +72,8 @@ func (api objectAPIHandlers) GetBucketNotificationHandler(w http.ResponseWriter,
 		writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL)
 		return
 	}
-	config.SetRegion(globalServerRegion)
-	if err = config.Validate(globalServerRegion, globalNotificationSys.targetList); err != nil {
+	config.SetRegion(globalSite.Region)
+	if err = config.Validate(globalSite.Region, globalNotificationSys.targetList); err != nil {
 		arnErr, ok := err.(*event.ErrARNNotFound)
 		if ok {
 			for i, queue := range config.QueueList {
@@ -145,7 +145,7 @@ func (api objectAPIHandlers) PutBucketNotificationHandler(w http.ResponseWriter,
 		return
 	}
 
-	config, err := event.ParseConfig(io.LimitReader(r.Body, r.ContentLength), globalServerRegion, globalNotificationSys.targetList)
+	config, err := event.ParseConfig(io.LimitReader(r.Body, r.ContentLength), globalSite.Region, globalNotificationSys.targetList)
 	if err != nil {
 		apiErr := errorCodes.ToAPIErr(ErrMalformedXML)
 		if event.IsEventError(err) {