hosts: Add dch-gw to zabbix group
The gateway device is now monitored by Zabbix. Adding it to the *zabbix* group ensures that the Zabbix agent is installed and configured correctly. Because the *zabbix-agent* role has a task to configure FirewallD, the `host_uses_firewalld` variable needs to be set to `false` for *gw0*, since it does not use FirewallD.
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