BindingGroupAddHandler should not validate the interface ref in the initial step

BindingGroupAddHandler does this in the initial step:

SocketBindingGroupResourceDefinition.validateDefaultInterfaceReference(context, model);

It actually adds a step to do other validation, so ^^^ should be moved there.

Doing this removes the requirement to run interface add ops before the socket-binding-group add ops, e.g. in non-xml-parsing based boots or in a post-boot composite operation.