Dependency on machine-id

[setrofim]

We rely on machine-id to generate the session ID (see: https://github.com/veraison/services/blob/main/verification/api/handler.go#L74). machine-id is a "systemd-ism", and this means that Veraison cannot be easily deployed on Linux systems not running systemd.

We need to either remove the dependency by figuring out some alternative way of getting a node ID (MAC address of the interface we're listening on? Generate our own node id as part of deployment?), or, at the very least, document the fact that the verification service expects /etc/machine-id to be present in the deployment documentation.