Fix the try_files directive generated in letsencrypt_provision_nginx_vhost_config

Problem/Motivation

According to the Nginx documentation when the HTTP code is returned, it should be preceded with an equal sign.

Currently every nginx vhost generated contains a few lines like
try_files $uri 404;

while it should be

try_files $uri =404;

Steps to reproduce

All virtual hosts contain slightly misconfigured try_files directive.

Proposed resolution

Update the letsencrypt_provision_nginx_vhost_config() function