Default is a random value between 0 and 6 hours. KVM and Slaprunner test instances set it at 0.

In the case of hellorina SR, the following section generated invalid command:

[proxy]
command-line = {{ python_location }}/bin/python2.7 {{ rina_proxy }} ${server:instance-guid} ${:ipv6} ${:port}
...

This value can be used as a unique identifier within the cloud.
It was already available for the requesting partition ('request' recipe).

/reviewed-on nexedi/slapos!101