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