move the dns_external check to using a group variable in the nagios group. This takes it out of the main inventory where its names do not match and this other group was not used in any other playbook

roles/nagios_server/templates/nagios/hostgroups/all-external.cfg.j2

@@ -1,7 +1,7 @@
 define hostgroup {
         hostgroup_name  dns
         alias           DNS Servers
-        members         {% for host in groups["dns_external"]|sort %} {{ hostvars[host]["inventory_hostname_short"] }}.fedoraproject.org {% if not loop.last %}, {% endif %} {% endfor %}
+        members         {% for host in vars['dns_external']|sort %}{{ host }}{% if not loop.last %}, {% endif %} {% endfor %}
 
 }