Consider validating interface refs on domain controller

The fix for the parent JIRA rejects refs to non-existent interfaces in socket-binding-group and socket-binding configs on servers. The check is not applied on the domain controller. Rationale for that is it is possible to defer specification of the interface to host.xml (where it would vary on each host), and not specify it at all at the domain level.

The configuration schema supports an "unspecified" interface config for domain.xml (i.e. the interface is just declared as a named placeholder with the requirement that the binding criteria exist in host.xml.) The design intent was this would be used in this sort of situation. So it's valid to do the reference check on the DC as well; I just didn't because I wasn't sure I was willing to break existing configs that didn't do things properly.