Allow configuring db_port for etherpad DB

etherpad/templates/settings.json.j2

@@ -19,10 +19,10 @@
     "password": "{{item.db_password}}",
     "database": "{{item.db_name}}",
     {% if item.db_type == "mysql" %}
-    "port": 3306,
+    "port": {{item.db_port|default(3306)}},
     "charset": "utf8mb4"
     {% elif item.db_type == "postgres" %}
-    "port": 5432
+    "port": {{item.db_port|default(5432)}}
     {% endif %}
   },