Set up auto registration of clients to Zabbix

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Currently Zabbix registers with the clients. Therefore we need the IP of the Autopower device for monitoring. This means that Zabbix monitoring doesn't work behind a NAT and requires manual setup. We should probably switch to Active Zabbix agents: https://blog.zabbix.com/zabbix-agent-active-vs-passive/9207/
Alternative: Auto registration: https://www.zabbix.com/documentation/current/en/manual/discovery/auto_registration#secure_autoregistration

[UsualSpec]

@romain-jacob I would need access to a device to implement this.

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Works with own device behind NAT, needs testing in the wild.

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Updates here:

I've updated Zabbix. I cannot confidently say that the issue is fixed. However, it is possible to at least pre-add the devices. No IP address needed and cloning an existing device is OK: This means: Selecting a device online in Zabbix, cloning it, removing the address. Afterwards, the device should auto-register.