DHCP scan using cluster interface name as network interface?

I'm working on IPv6 handling. A package built from fresh trunk gives me errors like:

Sep 5 13:33:24 maasboss maas.dhcp.detect: [ERROR] Ignoring DHCP scan for ipv6, it no longer exists. Check your cluster interfaces configuration.

My MAAS does have a cluster interface whose name is “ipv6” — but its *network* interface is “eth1”. The error comes from probe_interface, which is not exported, so you'd expect the caller to be within the module; but the only call there definitely uses the cluster interface's network interface, not its name.

However dhcp_probe_service now also calls probe_interface, and it passes the wrong value: it should pass interface['interface'] but instead it uses interface['name']. This won't show up in the default situation in an IPv4-only setup where users don't name their cluster interfaces, because there, the two strings will be identical.

So two things to fix:

1. Export probe_dhcp in __all__ (we ought to enforce this, or stop doing it, really).

2. Pass interface['interface'] to probe_dhcp, not interface['name'].