Use a variable for standardizing kubectl invocation (#8329)

* Add kubectl variable

* Replace kubectl usage by kubectl variable in roles

* Remove redundant --kubeconfig on kubectl usage

* Replace unecessary shell usage with command

roles/network_plugin/kube-ovn/tasks/main.yml

@@ -1,7 +1,7 @@
 ---
 - name: Kube-OVN | Label ovn-db node
   command: >-
-    {{ bin_dir }}/kubectl label --overwrite node {{ groups['kube_control_plane'] | first }} kube-ovn/role=master
+    {{ kubectl }} label --overwrite node {{ groups['kube_control_plane'] | first }} kube-ovn/role=master
   when:
     - inventory_hostname == groups['kube_control_plane'][0]