For machines that do not use firewalld, the *zabbix-agent* role will now
skip attempting to open the Zabbix agent port using the `firewalld`
module. The `host_uses_firewalld` variable controls this behavior.
The *zabbix-agent* role installs the Zabbix monitoring agent on the
managed node, and sets it up to communicate with the Zabbix server
specified by the `zabbix_server` variable. This role "should" be
compatible with most distributions; it has been tested with Fedora and
Gentoo.
