Add flexvolume plugin dir to kubeadm kubelet (#4168)

This was already approved in #4106 but there are CI issues
with that PR due to references to kubernetes incubator.

After upgrading to Kubespray 2.8.1 with Kubeadm enabled Rook
Ceph volume provision failed due to the flexvolume plugin dir not
being correct. Adding the var fixed the issue

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

@@ -121,6 +121,7 @@ KUBELET_NETWORK_PLUGIN="--network-plugin=cni --cni-conf-dir=/etc/cni/net.d --cni
 {% elif kube_network_plugin is defined and kube_network_plugin == "cloud" %}
 KUBELET_NETWORK_PLUGIN="--hairpin-mode=promiscuous-bridge --network-plugin=kubenet"
 {% endif %}
+KUBELET_VOLUME_PLUGIN="--volume-plugin-dir={{ kubelet_flexvolumes_plugins_dir }}"
 # Should this cluster be allowed to run privileged docker containers
 KUBE_ALLOW_PRIV="--allow-privileged=true"
 {% if cloud_provider is defined and cloud_provider in ["openstack", "azure", "vsphere", "aws"] %}