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-errors.go

@@ -396,10 +396,10 @@ func (e errorCodeMap) ToAPIErrWithErr(errCode APIErrorCode, err error) APIError
 	if err != nil {
 		apiErr.Description = fmt.Sprintf("%s (%s)", apiErr.Description, err)
 	}
-	if globalServerRegion != "" {
+	if globalSite.Region != "" {
 		switch errCode {
 		case ErrAuthorizationHeaderMalformed:
-			apiErr.Description = fmt.Sprintf("The authorization header is malformed; the region is wrong; expecting '%s'.", globalServerRegion)
+			apiErr.Description = fmt.Sprintf("The authorization header is malformed; the region is wrong; expecting '%s'.", globalSite.Region)
 			return apiErr
 		}
 	}
@@ -2286,7 +2286,7 @@ func getAPIErrorResponse(ctx context.Context, err APIError, resource, requestID,
 		BucketName: reqInfo.BucketName,
 		Key:        reqInfo.ObjectName,
 		Resource:   resource,
-		Region:     globalServerRegion,
+		Region:     globalSite.Region,
 		RequestID:  requestID,
 		HostID:     hostID,
 	}