Added external_port parameter

[blakejakopovic]

I'm using Gitlab that sit's behind an Nginx reverse proxy, and needed to change the external port. So I've added the external_port parameter. Note that to successfully use a different port, you need to add the port to the external_url as well. I'm currently using this fork, and it works as expected. I've also added a basic test. Please let me know if you have any concerns.

class { 'gitlab':
  external_url  => 'http://domain.name:5555',
  external_port => 5555,
}

[tobru]

Great! Thanks a lot for your contribution.