middleware/authpath: Fix return from k8s mw (#871)

* middleware/authpath: Fix return from k8s mw

Return the correct search path from the kubernetes' AutoPath function.
Based on preliminary discussion in #870

* PodWithIP can be private

Fix and add docs to functions.

* CR: remove the error from AutoPathFunc

middleware/kubernetes/kubernetes.go

@@ -47,6 +47,7 @@ type Kubernetes struct {
 	Fallthrough   bool
 
 	interfaceAddrsFunc func() net.IP
+	autoPathSearch     []string // Local search path from /etc/resolv.conf. Needed for autopath.
 }
 
 const (