install: foreign key for domain_ports

sql/mysql/domain_ports.sql

 CREATE TABLE `domain_ports` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
-  `id_domain` int(11) NOT NULL references domain(id),
+  `id_domain` int(11) NOT NULL references domains(id) on delete cascade,
   `public_port` int(11) DEFAULT NULL,
   `internal_port` int(11) DEFAULT NULL,
   `name` varchar(32) DEFAULT NULL,