Add support for configuring service backend
Add support for configuring service backend

On Fedora 23 there's no /var/log/secure, fail2ban must be configured to
read directly from journald. To do that we can just set backend =
systemd in service configuration. This change adds it to known
attributes that can be set through chef.

opscode-cookbooks/fail2ban,chef-cookbooks/fail2ban,opscode-cookbooks/fail2ban,chef-cookbooks/fail2ban