This new property be used as token to generate stable-ids instead
of the connection's UUID.

Later, this will be used by ipv6.addr-gen-mode=stable-privacy,
ethernet.cloned-mac-address=stable, and wifi.cloned-mac-address=stable
setting. Those generate stable addresses based on the connection's
UUID, but allow to use the stable-id instead.

This allows multiple connections to generate the same addresses
-- on the same machine, because in the above cases a machine
dependant key is also hashed.