* 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
