Maintain sticky flag on LB method validation failure.

We previously did not copy the sticky flag if the load-balancer
method validation failed, causing enabled stickiness to be dropped in
case of a validation error (which, technically, for us is the same as a
load-balancer configuration without an explicitly set method). This
change fixes that.

A few refactorings and improvements along the way:

- Move the frontend and backend configuration steps into separate
  methods/functions for better testability.
- Include the invalid method name in the error value and avoid log
  duplication.
- Add tests for the backend configuration part.

types/types.go

@@ -81,19 +81,18 @@ var loadBalancerMethodNames = []string{
 
 // NewLoadBalancerMethod create a new LoadBalancerMethod from a given LoadBalancer.
 func NewLoadBalancerMethod(loadBalancer *LoadBalancer) (LoadBalancerMethod, error) {
+	var method string
 	if loadBalancer != nil {
+		method = loadBalancer.Method
 		for i, name := range loadBalancerMethodNames {
-			if strings.EqualFold(name, loadBalancer.Method) {
+			if strings.EqualFold(name, method) {
 				return LoadBalancerMethod(i), nil
 			}
 		}
 	}
-	return Wrr, ErrInvalidLoadBalancerMethod
+	return Wrr, fmt.Errorf("invalid load-balancing method '%s'", method)
 }
 
-// ErrInvalidLoadBalancerMethod is thrown when the specified load balancing method is invalid.
-var ErrInvalidLoadBalancerMethod = errors.New("Invalid method, using default")
-
 // Configuration of a provider.
 type Configuration struct {
 	Backends  map[string]*Backend  `json:"backends,omitempty"`