Executor: Use DHCP requests to make the discovery process faster

Now that the DUTs' DHCP server is in control of the executor, we could reduce the discovery process timeout by associating a PDU port to the mac address of the new machine upon its first DHCP request.

This would reduce the amount of components that need to work before a machine gets registered (linux, b2c, registry, gfxinfo, machine_registration, ...), while also making it quicker to see the association in the dashboard.

One slight issue though: the association will list a pretty bare-boned machine... no tags or anything. So we'll need to be ready for this and just mark it in the dashboard as "Waiting for initial provisioning" or something like that!