[2.1.1 UI] rdns for discovered devices should also preselect domain

[Mark Shuttleworth]

When device discovery includes a name with a domain ("foo.mallards") and the domain is one of those known to MAAS, then it would be great to pre-select that domain in the expanded "register a host" view of that item in the discovered nodes list. The hostname should be just the hostname, and the domain pre-selected.

So if a node called "foo.mallards" is observed, and there is a domain called "mallards" in MAAS, then expanding the discovered node line should give me "foo" as a suggested hostname and pre-select the "mallards" domain.