Use K8s 1.15 (#4905)

* Use K8s 1.15

* Use Kubernetes 1.15 and use kubeadm.k8s.io/v1beta2 for
  InitConfiguration.
* bump to v1.15.0

* Remove k8s 1.13 checksums.

* Update README kubernetes version 1.15.0.

* Update metrics server 0.3.3 for k8s 1.15

* Remove less than k8s 1.14 related code

* Use kubeadm with --upload-certs instead of --experimental-upload-certs due to depricate

* Update dnsautoscaler 1.6.0

* Skip certificateKey if it's not defined

* Add kubeadm-conftolplane.v2beta2 for k8s 1.15 or later

* Support kubeadm control plane for k8s 1.15

* Update sonobuoy version 0.15.0 for k8s 1.15

roles/kubernetes/node/tasks/kubelet.yml

@@ -29,17 +29,6 @@
     - kubelet
     - kubeadm
 
-- name: Write kubelet environment config file (kubeadm)
-  template:
-    src: "kubelet.env.j2"
-    dest: "{{ kube_config_dir }}/kubelet.env"
-    backup: yes
-  notify: restart kubelet
-  when: kubeadm_output.stdout is version('v1.13.0', '<')
-  tags:
-    - kubelet
-    - kubeadm
-
 - name: Write kubelet config file
   template:
     src: "kubelet-config.{{ kubeadmConfig_api_version }}.yaml.j2"