Add cert-manager and let's encrypt to the k8s-cluster

This change contains a new module which will be applied after the
kubernetes-cluster has been created. It will install the cert-manager in
it and add let's encrypt as a ClusterIssuer to the cluster. That setup
allows me to simply issue certificates for all services in the cluster.

k8s/variables.tf

@@ -0,0 +1,3 @@
+variable "letsencrypt_issuer_email" {
+  type = string
+}