roles/nginx: Update nginx.conf.j2

This is to accomodate Debian's 9 nginx package, as it provides a
different system user/group than nginx.org's packages.

roles/nginx/templates/nginx.conf.j2

@@ -1,6 +1,10 @@
 
 # Run as a unique, less privileged user for security reasons.
+{% if ansible_distribution == 'Debian' and ansible_distribution_major_version | version_compare('9', '==') %}
+user  www-data;
+{% else %}
 user nginx;
+{% endif %}
 
 # Sets the worker threads to the number of CPU cores available in the system for best performance.
 # Should be > the number of CPU cores.