Allow configuration of nginx_ssl_directory

[cristoper]

For my use case it will likely be convenient to be able to configure the directory where the SSL certificate/key is located (instead of hardcoding it to /etc/nginx/ssl). I have only very quickly tested this patch.

[nickjj]

Thanks for the contribution.

I am currently in the process of rewriting both this role and my let's encrypt role.

A configurable SSL location and multiple certificates (at the LE level) are on the list.

Going to keep this PR open as a reminder until they are both ready.

[cristoper]

Cool (I just submitted a small PR to your LE role also). Both these Ansible roles have already saved me time, so I look forward to the improved versions!