Update module github.com/kubernetes-incubator/external-dns to v0.8.0

Author: renovate-bot

go.mod

@@ -9,7 +9,7 @@ require (
 	github.com/gophercloud/gophercloud v0.14.0
 	github.com/imdario/mergo v0.3.7 // indirect
 	github.com/jetstack/cert-manager v0.16.1
-	github.com/kubernetes-incubator/external-dns v0.5.12
+	github.com/kubernetes-incubator/external-dns v0.8.0
 	github.com/sirupsen/logrus v1.7.0
 	golang.org/x/oauth2 v0.0.0-20191202225959-858c2ad4c8b6 // indirect
 	golang.org/x/time v0.0.0-20191024005414-555d28b269f0 // indirect