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/vars/CentOS-8.yml

@@ -1,2 +1,3 @@
+nginx_user: nginx
 nginx_default_ssl_ciphers:
 - PROFILE=SYSTEM

roles/nginx/vars/Fedora.yml

@@ -1,2 +1,3 @@
+nginx_user: nginx
 nginx_default_ssl_ciphers:
 - PROFILE=SYSTEM

roles/nginx/vars/defaults.yml

@@ -1,3 +1,4 @@
+nginx_user: www-data
 nginx_default_ssl_ciphers:
 - HIGH
 - '!aNULL'