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/kubernetes/control-plane/tasks/kubeadm-setup.yml

@@ -190,7 +190,7 @@
 
 # FIXME(mattymo): from docs: If you don't want to taint your control-plane node, set this field to an empty slice, i.e. `taints: {}` in the YAML file.
 - name: kubeadm | Remove taint for master with node role
-  command: "{{ bin_dir }}/kubectl --kubeconfig {{ kube_config_dir }}/admin.conf taint node {{ inventory_hostname }} {{ item }}"
+  command: "{{ kubectl }} taint node {{ inventory_hostname }} {{ item }}"
   delegate_to: "{{ first_kube_control_plane }}"
   with_items:
     - "node-role.kubernetes.io/master:NoSchedule-"

roles/kubernetes/control-plane/tasks/kubeadm-upgrade.yml

@@ -61,8 +61,7 @@
 # FIXME: https://github.com/kubernetes/kubeadm/issues/1318
 - name: kubeadm | scale down coredns replicas to 0 if not using coredns dns_mode
   command: >-
-    {{ bin_dir }}/kubectl
-    --kubeconfig {{ kube_config_dir }}/admin.conf
+    {{ kubectl }}
     -n kube-system
     scale deployment/coredns --replicas 0
   register: scale_down_coredns