kubernetes: Manage worker nodes

So far, I have been managing Kubernetes worker nodes with Fedora CoreOS
Ignition, but I have decided to move everything back to Fedora and
Ansible.  I like the idea of an immutable operating system, but the FCOS
implementation is not really what I want.  I like the automated updates,
but that can be accomplished with _dnf-automatic_.  I do _not_ like
giving up control of when to upgrade to the next Fedora release.
Mostly, I never did come up with a good way to manage application-level
configuration on FCOS machines.  None of my experiments (Cue+tmpl,
KCL+etcd+Luci) were successful, which mostly resulted in my manually
managing configuration on nodes individually.  Managing OS-level
configuration is also rather cumbersome, since it requires redeploying
the machine entirely.  Altogether, I just don't think FCOS fits with my
model of managing systems.

This commit introduces a new playbook, `kubernetes.yml`, and a handful of
new roles to manage Kubernetes worker nodes running Fedora Linux.  It
also adds two new deploy scripts, `k8s-worker.sh` and `k8s-longhorn.sh`,
which fully automate the process of bringing up worker nodes.

group_vars/k8s-longhorn.yml

@@ -0,0 +1,18 @@
+data_volumes:
+- dev: /dev/vdb
+  fstype: ext4
+  mountpoint: /var/lib/containers
+- dev: /dev/vdc
+  fstype: ext4
+  mountpoint: /var/lib/kubelet
+- dev: /dev/vdd
+  fstype: ext4
+  mountpoint: /var/lib/longhorn
+
+k8s_node_labels:
+  node-role.kubernetes.io/longhorn: ''
+  node.longhorn.io/create-default-disk: 'true'
+
+k8s_node_taints:
+- key: node-role.kubernetes.io/longhorn
+  effect: NoSchedule