zabbix_agentd: Add OpenVPN certificates items

zabbix_agentd: Add OpenVPN certificates items

- Adds Zabbix Agent userparameters `ipfire.ovpn.clientcert` and `ipfire.ovpn.cacert` for the agent to get details about openvpn client, server and ca certificates.
- Moves all `ipfire.ovpn.*` userparameters to a separate config file `userparameter_ovpn.conf` to enable users to selectively disable openvpn items when not needed
- Includes `ipfire_certificate_detail.sh` script in sudoers for Zabbix Agent as it needs root permission to read openvpn certificate details.
- Adapts lfs install script to install new script and configfile
- Adds new script and configfile to rootfiles
Reviewed-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>