fixes roles/docker/vars/ubuntu-bionic.yml points to xenial (#3395)
fixes roles/docker/vars/ubuntu-bionic.yml points to xenial (#3395)

* fixes: #3387
kubernetes-incubator/kargo,kubernetes-sigs/kubespray,insequent/kargo,kubernetes-sigs/kubespray,Atoms/kubespray,kubernetes-sigs/kubespray,kubespray/kargo,kubernetes-sigs/kubespray,kubernetes-incubator/kargo,kubespray/kargo,Atoms/kubespray,Atoms/kubespray,kubernetes-incubator/kubespray,kubernetes-incubator/kubespray,Atoms/kubespray,insequent/kargo