Define a standard commandline for applying manifests

This is expected to be used in the command module this way:
command:
  cmd: "{{ kubectl_apply_stdin }}"
  stdin: <... rendered manifests > -> using the 'template' lookup plugin
  in most cases.

The advantages over the kube plugin module integrated in kubespray
(which this should replace eventually):
- way easier to modify to take advantage of new features (server-side
  apply for instance)
- no need for a separate template tasks + checking the result (which can
  introduce problem if the first playbook runs encounters an error).

roles/kubernetes-apps/defaults/main.yml

@@ -0,0 +1,2 @@
+---
+namespace: kube-system

roles/kubernetes-apps/vars/main.yml

@@ -0,0 +1,2 @@
+---
+kubectl_apply_stdin: "{{ kubectl }} apply -f - -n {{ namespace }}"