Allow setting cgroup driver for kubelet

Red Hat family platforms run docker daemon with `--exec-opt
native.cgroupdriver=systemd`. When kubespray tried to start kubelet
service, it failed with:

Error: failed to run Kubelet: failed to create kubelet: misconfiguration: kubelet cgroup driver: "cgroupfs" is different from docker cgroup driver: "systemd"

Setting kubelet's cgroup driver to the correct value for the platform
fixes this issue. The code utilizes autodetection of docker's cgroup
driver, as different RPMs for the same distro may vary in that regard.

roles/kubernetes/node/tasks/facts.yml

@@ -0,0 +1,8 @@
+- name: look up docker cgroup driver
+  shell: "docker info | grep 'Cgroup Driver' | awk -F': ' '{ print $2; }'"
+  register: docker_cgroup_driver_result
+
+- set_fact:
+    standalone_kubelet: >-
+      {%- if inventory_hostname in groups['kube-master'] and inventory_hostname not in groups['kube-node'] -%}true{%- else -%}false{%- endif -%}
+    kubelet_cgroup_driver_detected: "{{ docker_cgroup_driver_result.stdout }}"

roles/kubernetes/node/tasks/main.yml

@@ -1,7 +1,5 @@
 ---
-- set_fact:
-    standalone_kubelet: >-
-      {%- if inventory_hostname in groups['kube-master'] and inventory_hostname not in groups['kube-node'] -%}true{%- else -%}false{%- endif -%}
+- include: facts.yml
   tags: facts
 
 - include: pre_upgrade.yml

roles/kubernetes/node/templates/kubelet.standard.env.j2

@@ -19,6 +19,7 @@ KUBELET_HOSTNAME="--hostname-override={{ kube_override_hostname }}"
 {% if kube_version | version_compare('v1.7', '<') %}
 --enable-cri={{ kubelet_enable_cri }} \
 {% endif %}
+--cgroup-driver={{ kubelet_cgroup_driver|default(kubelet_cgroup_driver_detected) }} \
 --cgroups-per-qos={{ kubelet_cgroups_per_qos }} \
 --enforce-node-allocatable={{ kubelet_enforce_node_allocatable }} {% endif %}{% endset %}