Get port from parameter instead of reading it from PORT file

Reading from the PORT file was not working for current version of Dokku as it seems to be updated after the hook. So I have modified it to use the port that come in the parameter as may be observed [here](https://github.com/progrium/dokku/blob/4e9d797d452d9e8499eec67ffad3e4140fc22736/plugins/nginx-vhosts/post-deploy#L92).