r/collectd-prometheus: Work w/o firewalld, selinux

Tasks that configure the SELinux policy obviously only make sense if the
host uses SELinux.  Similarly, if the host does not use FirewallD,
configuring firewall rules doesn't work.

roles/collectd-prometheus/tasks/main.yml

@@ -21,6 +21,7 @@
     dest: /var/lib/selinux/collectd-local.pp
   notify:
   - install collectd-local selinux module
+  when: ansible_selinux.status == 'enabled'
   tags:
   - selinux