k8s 1.13, knative and many

[crystaldust]

The PR includes many new changes:

• Add go.mod file, make it easier to handle the dependencies with vgo or go 1.11
• Add templates for kubernetes 1.13 and etcd 3.3.10
• Update the README.md file, add the latest supported k8s version and binaries download URLs
• Add the --requestheader-client-ca-file for kube-apiserver 1.10 or later, make the cluster support knative by default.