add a way to include custom ssl_dhparam file about ansible-role-nginx

it's possible to generate one on the fly, but it's kinda slow (on my machine at least), so i'm not sure if we should do that by default

the only way i can get this to work is by installing nginx first. Once it's installed, then i have the right permissions to load the ssl certs and ssl_dhparam file.

[jdauphant]

I forgot it was made on my other role that can combine : https://github.com/jdauphant/ansible-role-ssl-certs#example-to-use-this-role-with-my-nginx-role-jdauphantnginx