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/server-startup-msg.go

@@ -126,7 +126,7 @@ func printServerCommonMsg(apiEndpoints []string) {
 	cred := globalActiveCred
 
 	// Get saved region.
-	region := globalServerRegion
+	region := globalSite.Region
 
 	apiEndpointStr := strings.Join(apiEndpoints, "  ")