Otherwise, the generated client-id depends purely on the profile's
stable-id. It means, the same profile (that is, either the same UUID
or same stable-id) on different hosts will result in identical client-ids.

That is clearly not desired. Hash a per-host secret-key as well.

Note, that we don't hash the interface name. So, activating the
profile on different interfaces, will still yield the same client-id.
But also note, that commonly a profile is restricted to one device,
via "connection.interface-name".

Note that this is a change in behavior. However, "ipv4.dhcp-client-id=stable"
was only added recently and not yet released.

Fixes: 62a78639