Add default ports for both engines

[egegunes]

if you don't specify the port, it fails to connect.

$ random-data-load run --engine=mysql --user=root --host=127.0.0.1 --password='<redacted>' --database=test --table=random --rows=20000
random-data-load: error: LoadTable: dial tcp 127.0.0.1:0: connect: connection refused