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.

roles/k8s-worker/tasks/main.yml

@@ -0,0 +1,30 @@
+- name: stat /var/lib/kubelet/config.yaml
+  stat:
+    path: /var/lib/kubelet/config.yaml
+  ignore_errors: true
+  register: stat_kublet_config
+  tags:
+  - kubeadm-join
+
+- name: generate bootstrap token
+  delegate_to: '{{ groups["k8s-controller"][0] }}'
+  command:
+    kubeadm token create
+    --kubeconfig /etc/kubernetes/admin.conf
+    --ttl 1h
+    --print-join-command
+  when:
+    not stat_kublet_config.stat.exists
+  changed_when: true
+  register: kubeadm_token_create
+  tags:
+  - bootstrap-token
+  - kubeadm-join
+- name: join the kubernetes cluster
+  command: >-
+    {{ kubeadm_token_create.stdout }}
+  when:
+    not stat_kublet_config.stat.exists
+  changed_when: true
+  tags:
+  - kubeadm-join