Add an option to specify MySQL DB Port

[jloh]

When installing Ghost there isn't an option to specify a custom database port for MySQL. If you're installing it on a managed database service that runs on a non-standard MySQL port this means you have to install ghost initially and let it fail, then manually edit the config.production.json file and add the port, then restart ghost.

[Charizard]

I would like to pick this up.