Add utils for retrieving backendconfigs for service (and reversely)

[MrHohn]

/assign @rramkumar1 @nicksardo
cc @bowei

[MrHohn]

Planing to plug this to trigger Ingress sync.

[MrHohn]

Planing to plug this into servicePort():

ingress-gce/pkg/backends/utils.go

Lines 50 to 95 in 16d4f39

	func servicePort(ib v1beta1.IngressBackend, svc v1.Service) (ServicePort, error) {
	// If service is not of type NodePort, return an error.
	if svc.Spec.Type != v1.ServiceTypeNodePort {
	return ServicePort{}, fmt.Errorf("service %v/%v for backend %+v is type %v, expected type NodePort", svc.Namespace, svc.Name, ib, svc.Spec.Type)
	}
	appProtocols, err := annotations.FromService(&svc).ApplicationProtocols()
	if err != nil {
	return ServicePort{}, err
	}
	var port *v1.ServicePort
	PortLoop:
	for _, p := range svc.Spec.Ports {
	np := p
	switch ib.ServicePort.Type {
	case intstr.Int:
	if p.Port == ib.ServicePort.IntVal {
	port = &np
	break PortLoop
	}
	default:
	if p.Name == ib.ServicePort.StrVal {
	port = &np
	break PortLoop
	}
	}
	}
	if port == nil {
	return ServicePort{}, fmt.Errorf("could not find matching port on service %v/%v for backend %+v. Looking for port %+v in %v", svc.Namespace, ib.ServiceName, ib, ib.ServicePort, svc.Spec.Ports)
	}
	proto := annotations.ProtocolHTTP
	if protoStr, exists := appProtocols[port.Name]; exists {
	glog.V(2).Infof("service %v/%v, port %q: using protocol to %q", svc.Namespace, ib.ServiceName, port, protoStr)
	proto = annotations.AppProtocol(protoStr)
	}
	p := ServicePort{
	NodePort: int64(port.NodePort),
	Protocol: proto,
	SvcName: types.NamespacedName{Namespace: svc.Namespace, Name: ib.ServiceName},
	SvcPort: ib.ServicePort,
	}
	return p, nil
	}

.

[rramkumar1]

/lgtm
Leaving to @nicksardo for approval.

[MrHohn]

Thanks for the review!