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/config.go

@@ -186,11 +186,6 @@ func readServerConfig(ctx context.Context, objAPI ObjectLayer) (config.Config, e
 // ConfigSys - config system.
 type ConfigSys struct{}
 
-// Load - load config.json.
-func (sys *ConfigSys) Load(objAPI ObjectLayer) error {
-	return sys.Init(objAPI)
-}
-
 // Init - initializes config system from config.json.
 func (sys *ConfigSys) Init(objAPI ObjectLayer) error {
 	if objAPI == nil {