r/nginx: Fix applying on Buildroot systems

There are a few minor differences between the way Fedora and Buildroot
package *nginx*:

* Fedora uses a user named *nginx* while buildroot uses *www-data*
* Buildroot uses a Debian-like configuration layout (with
  `sites-enabled` and `modules-enabled` directories)

This commit adjusts the *nginx* Ansible role to compensate for these
differences, eschewing Buildroot's configuration layout for the one used
by Fedora/Red Hat.

roles/nginx/templates/nginx.conf.j2

@@ -2,7 +2,7 @@
 #   * Official English Documentation: http://nginx.org/en/docs/
 #   * Official Russian Documentation: http://nginx.org/ru/docs/
 
-user nginx;
+user {{ nginx_user }};
 worker_processes auto;
 error_log /var/log/nginx/error.log;
 {% if nginx_log_syslog|bool %}