Move _as_new_style_instance_details out from tests

This function was originally written for the tests, but ended up being
useful for the SUT as well. Thanks, @cyli for catching this refactoring
opportunity!
Tweak line ordering

This relatively minor change makes it clearer that a particular section
is all about CLB, given that `remove_from_load_balancer` will be a
backend-agnostic API.