implement canary annotation and alternative backends

Adds the ability to create alternative backends. Alternative backends enable
traffic shaping by sharing a single location but routing to different
backends depending on the TrafficShapingPolicy defined by AlternativeBackends.

When the list of upstreams and servers are retrieved, we then call
mergeAlternativeBackends which iterates through the paths of every ingress
and checks if the backend supporting the path is a AlternativeBackend. If
so, we then iterate through the map of servers and find the real backend
that the AlternativeBackend should fall under. Once found, the
AlternativeBackend is embedded in the list of VirtualBackends for the real
backend.

If no matching real backend for a AlternativeBackend is found, then the
AlternativeBackend is deleted as it cannot be backed by any server.

internal/ingress/errors/errors.go

@@ -32,6 +32,14 @@ var (
 	ErrInvalidAnnotationName = errors.New("invalid annotation name")
 )
 
+// NewInvalidAnnotationConfiguration returns a new InvalidConfiguration error for use when
+// annotations are not correctly configured
+func NewInvalidAnnotationConfiguration(name string, reason string) error {
+	return InvalidConfiguration{
+		Name: fmt.Sprintf("the annotation %v does not contain a valid configuration: %v", name, reason),
+	}
+}
+
 // NewInvalidAnnotationContent returns a new InvalidContent error
 func NewInvalidAnnotationContent(name string, val interface{}) error {
 	return InvalidContent{
@@ -46,6 +54,15 @@ func NewLocationDenied(reason string) error {
 	}
 }
 
+// InvalidConfiguration Error
+type InvalidConfiguration struct {
+	Name string
+}
+
+func (e InvalidConfiguration) Error() string {
+	return e.Name
+}
+
 // InvalidContent error
 type InvalidContent struct {
 	Name string