Fix the condition of drain on pre-remove task (#8634)

When running cluster.yml for new machines what containerd is already
install but Kubernetes cluster were not installed before, the task
"remove-node | List nodes" is failed like

  "changed": false,
  "cmd": [
    "/usr/local/bin/kubectl", "--kubeconfig",
    "/etc/kubernetes/admin.conf", "get", "nodes", "-o",
    "go-template={{ range .items }}{{ .metadata.name }}
    {{ "\n" }}{{ end }}"
   ],
   ..
   "stderr": "error: stat /etc/kubernetes/admin.conf: no such file or directory",

That was due to lack to check the existing Kubernetes cluster exists
or not before running "kubectl drain" command.
This adds the check to avoid the issue.

roles/container-engine/validate-container-engine/tasks/main.yml

@@ -63,6 +63,8 @@
         apply:
           tags:
             - pre-remove
+      when:
+        - kubelet_systemd_unit_exists
     - name: Stop kubelet
       service:
         name: kubelet
@@ -89,6 +91,8 @@
         apply:
           tags:
             - pre-remove
+      when:
+        - kubelet_systemd_unit_exists
     - name: Stop kubelet
       service:
         name: kubelet
@@ -114,6 +118,8 @@
         apply:
           tags:
             - pre-remove
+      when:
+        - kubelet_systemd_unit_exists
     - name: Stop kubelet
       service:
         name: kubelet