Update module jetstack/cert-manager to v0.16.1

Author: renovate-bot

go.mod

@@ -8,13 +8,10 @@ require (
 	github.com/google/go-cmp v0.3.1 // indirect
 	github.com/gophercloud/gophercloud v0.12.0
 	github.com/imdario/mergo v0.3.7 // indirect
-	github.com/jetstack/cert-manager v0.15.0
+	github.com/jetstack/cert-manager v0.16.1
 	github.com/kubernetes-incubator/external-dns v0.5.12
-	github.com/miekg/dns v1.0.8 // indirect
 	github.com/sirupsen/logrus v1.6.0
-	golang.org/x/net v0.0.0-20191209160850-c0dbc17a3553 // indirect
 	golang.org/x/oauth2 v0.0.0-20191202225959-858c2ad4c8b6 // indirect
-	golang.org/x/sys v0.0.0-20191228213918-04cbcbbfeed8 // indirect
 	golang.org/x/time v0.0.0-20191024005414-555d28b269f0 // indirect
 	google.golang.org/appengine v1.6.5 // indirect
 	google.golang.org/genproto v0.0.0-20191230161307-f3c370f40bfb // indirect