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/handler-utils.go

@@ -58,7 +58,7 @@ func parseLocationConstraint(r *http.Request) (location string, s3Error APIError
 	} // else for both err as nil or io.EOF
 	location = locationConstraint.Location
 	if location == "" {
-		location = globalServerRegion
+		location = globalSite.Region
 	}
 	return location, ErrNone
 }
@@ -66,7 +66,7 @@ func parseLocationConstraint(r *http.Request) (location string, s3Error APIError
 // Validates input location is same as configured region
 // of MinIO server.
 func isValidLocation(location string) bool {
-	return globalServerRegion == "" || globalServerRegion == location
+	return globalSite.Region == "" || globalSite.Region == location
 }
 
 // Supported headers that needs to be extracted.
@@ -222,7 +222,7 @@ func extractReqParams(r *http.Request) map[string]string {
 		return nil
 	}
 
-	region := globalServerRegion
+	region := globalSite.Region
 	cred := getReqAccessCred(r, region)
 
 	principalID := cred.AccessKey