Add default port for aurora (#4102)

If a port isn't specified, it's looked up. The lookup breaks without
this.

Related: https://github.com/ansible/ansible-modules-core/pull/3414

lib/ansible/modules/cloud/amazon/rds.py

@@ -320,6 +320,7 @@ except ImportError:
     has_rds2 = False
 
 DEFAULT_PORTS= {
+    'aurora': 3306,
     'mariadb': 3306,
     'mysql': 3306,
     'oracle': 1521,