Remove istio support

inventory/sample/group_vars/k8s-cluster.yml

roles/download/defaults/main.yml

roles/kubernetes-apps/istio/defaults/main.yml

roles/kubernetes-apps/istio/tasks/main.yml

.../kubernetes-apps/istio/templates/istio-initializer.yml.j2

roles/kubernetes-apps/istio/templates/istio.yml.j2

roles/kubernetes-apps/meta/main.yml

...s/kubernetes/preinstall/tasks/growpart-azure-centos-7.yml

roles/kubespray-defaults/defaults/main.yaml

tests/files/gce_centos7-flannel-addons.yml