Use attribute for listen port in nginx default server template

Deals with the same issue that opscode-cookbooks/nginx#201 offers to provide, but following Chef convention more closely (as per @eherot’s [comment on the abovementioned PR](https://github.com/opscode-cookbooks/nginx/pull/201/files#r12709685)).
Wouldn't this be a change, which breaks backwards compatibility? Currently the used port for the default page is `80`, which would be changed to `8080` by this ...

Apart from that, I am :+1: on making the default port be read from an attribute.