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-response.go

@@ -787,9 +787,9 @@ func writeErrorResponse(ctx context.Context, w http.ResponseWriter, err APIError
 		// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After
 		w.Header().Set(xhttp.RetryAfter, "120")
 	case "InvalidRegion":
-		err.Description = fmt.Sprintf("Region does not match; expecting '%s'.", globalServerRegion)
+		err.Description = fmt.Sprintf("Region does not match; expecting '%s'.", globalSite.Region)
 	case "AuthorizationHeaderMalformed":
-		err.Description = fmt.Sprintf("The authorization header is malformed; the region is wrong; expecting '%s'.", globalServerRegion)
+		err.Description = fmt.Sprintf("The authorization header is malformed; the region is wrong; expecting '%s'.", globalSite.Region)
 	}
 
 	// Generate error response.