Implement optional deployment of cert-manager to testcluster.

Created by: garloff

This addresses #116 (closed). We follow https://cert-manager.io/v1.1-docs/usage/kubectl-plugin/. We have: apply_cert-manager.sh, which is called by create_cluster.sh IF DEPLOY_CERT_MANAGER == "true" (passed down via deploy_cert_manager). This also installs the kubectl_cert_manager plugin into /usr/local/bin (though admittedly, this is usually more interesting for the cluster-admin and not the provider).

We also have cert-manager-test.yaml to create a test issuer. Can be used for testing, not yet hooked to any automated CI test.

Signed-off-by: Kurt Garloff kurt@garloff.de