zabbix_agentd: Add ovpn monitoring items

zabbix_agentd: Add ovpn monitoring items

Added new IPFire specific monitoring capabilities to Zabbix Agent:
- ipfire.ovpn.clients.discovery: Discovery of configured ovpn
  clients. Returns a JSON array.
- ipfire.ovpn.statusreport.get: Parses and returns
  /var/run/ovpnserver.log in a JSON array

Since /var/run/ovpnserver.log is only readable by root, 'cat' of that
file is added to sudoers.d/zabbix_agentd.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>