remove tests for ubuntu 19.x (EOL). Add ubuntu 18.04
remove tests for ubuntu 19.x (EOL). Add ubuntu 18.04

warmchang/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,kubernetes/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube,warmchang/minikube,kubernetes/minikube,kubernetes/minikube