middleware/kubernetes: cleanup (#818)

Drop the interfaceAddr interfaces and just use a function. Cleanup
all fallout from that. Remove the use of global variables and cleanup
the tests a bit.

middleware/kubernetes/kubernetes_test.go

@@ -432,6 +432,7 @@ func TestServices(t *testing.T) {
 
 	k := Kubernetes{Zones: []string{"interwebs.test"}}
 	k.Federations = []Federation{{name: "fed", zone: "era.tion.com"}}
+	k.interfaceAddrsFunc = localPodIP
 	k.APIConn = &APIConnServiceTest{}
 
 	type svcAns struct {