kubeadm: update HA etcd guide for clarity and fix an issue

There were a couple of reported problems with this guide:

- The introductory paragraph talks about single control plane nodes
and does not mention the different options for HA etcd. Clear the
language to reduce the confusion and cross-link to the ha-topology
page.
- The hostname / IP detection in kubeadm can end
up with values not suitable for the certificates that kubeadm
generates for all etcd instances. Ensure that the hostnames / IPs
are pinned by the user in the example script.

Side cleanup related to the dockershim removal:
- Use containerd in the setup example and don't mention docker as
a requirement.

Author: neolit123

content/en/docs/setup/production-environment/tools/kubeadm/setup-ha-etcd-with-kubeadm.md

@@ -16,19 +16,20 @@ or upgrades for such nodes. The long term plan is to empower the tool
 aspects.
 {{< /note >}}
 
-Kubeadm defaults to running a single member etcd cluster in a static pod managed
-by the kubelet on the control plane node. This is not a high availability setup
-as the etcd cluster contains only one member and cannot sustain any members
-becoming unavailable. This task walks through the process of creating a high
-availability etcd cluster of three members that can be used as an external etcd
-when using kubeadm to set up a kubernetes cluster.
+By default, kubeadm runs a local etcd instance on each control plane node.
+It is also possible to treat the etcd cluster as external and provision
+etcd instances on separate hosts. The differences between the two approaches are covered in the
+[Options for Highly Available topology][/docs/setup/production-environment/tools/kubeadm/ha-topology] page.
+This task walks through the process of creating a high availability external
+etcd cluster of three members that can be used by kubeadm during cluster creation.
 
 ## {{% heading "prerequisites" %}}
 
-* Three hosts that can talk to each other over ports 2379 and 2380. This
+* Three hosts that can talk to each other over TCP ports 2379 and 2380. This
   document assumes these default ports. However, they are configurable through
   the kubeadm config file.
-* Each host must [have docker, kubelet, and kubeadm installed](/docs/setup/production-environment/tools/kubeadm/install-kubeadm/).
+* Each host must have systemd and a bash compatible shell installed.
+* Each host must [have a container runtime, kubelet, and kubeadm installed](/docs/setup/production-environment/tools/kubeadm/install-kubeadm/).
 * Each host should have access to the Kubernetes container image registry (`k8s.gcr.io`) or list/pull the required etcd image using
 `kubeadm config images list/pull`. This guide will setup etcd instances as
 [static pods](/docs/tasks/configure-pod-container/static-pod/) managed by a kubelet.
@@ -48,6 +49,11 @@ the certificates described below; no other cryptographic tooling is required for
 this example.
 {{< /note >}}
 
+{{< note >}}
+The examples below use IPv4 addresses but you can also configure kubeadm, the kubelet and etcd
+to use IPv6 addresses. Dual-stack is supported by some Kubernetes options, but not by etcd. For more details
+on Kubernetes dual-stack support see [Dual-stack support with kubeadm](/docs/setup/production-environment/tools/kubeadm/dual-stack-support/).
+{{< /note >}}
 
 1. Configure the kubelet to be a service manager for etcd.
 
@@ -59,8 +65,9 @@ this example.
     cat << EOF > /etc/systemd/system/kubelet.service.d/20-etcd-service-manager.conf
     [Service]
     ExecStart=
-    #  Replace "systemd" with the cgroup driver of your container runtime. The default value in the kubelet is "cgroupfs".
-    ExecStart=/usr/bin/kubelet --address=127.0.0.1 --pod-manifest-path=/etc/kubernetes/manifests --cgroup-driver=systemd
+    # Replace "systemd" with the cgroup driver of your container runtime. The default value in the kubelet is "cgroupfs".
+    # Replace the value of "--container-runtime-endpoint" for a different container runtime if needed.
+    ExecStart=/usr/bin/kubelet --address=127.0.0.1 --pod-manifest-path=/etc/kubernetes/manifests --cgroup-driver=systemd --container-runtime=remote --container-runtime-endpoint=unix:///var/run/containerd/containerd.sock
     Restart=always
     EOF
 
@@ -80,21 +87,34 @@ this example.
     member running on it using the following script.
 
     ```sh
-    # Update HOST0, HOST1, and HOST2 with the IPs or resolvable names of your hosts
+    # Update HOST0, HOST1 and HOST2 with the IPs of your hosts
     export HOST0=10.0.0.6
     export HOST1=10.0.0.7
     export HOST2=10.0.0.8
 
+    # Update NAME0, NAME1 and NAME2 with the hostnames of your hosts
+    export NAME0="infra0"
+    export NAME1="infra1"
+    export NAME2="infra2"
+
     # Create temp directories to store files that will end up on other hosts.
     mkdir -p /tmp/${HOST0}/ /tmp/${HOST1}/ /tmp/${HOST2}/
 
-    ETCDHOSTS=(${HOST0} ${HOST1} ${HOST2})
-    NAMES=("infra0" "infra1" "infra2")
+    HOSTS=(${HOST0} ${HOST1} ${HOST2})
+    NAMES=(${NAME0} ${NAME1} ${NAME2})
 
-    for i in "${!ETCDHOSTS[@]}"; do
-    HOST=${ETCDHOSTS[$i]}
+    for i in "${!HOSTS[@]}"; do
+    HOST=${HOSTS[$i]}
     NAME=${NAMES[$i]}
     cat << EOF > /tmp/${HOST}/kubeadmcfg.yaml
+    ---
+    apiVersion: "kubeadm.k8s.io/v1beta3"
+    kind: InitConfiguration
+    nodeRegistration:
+        name: ${NAME}
+    localAPIEndpoint:
+        advertiseAddress: ${HOST}
+    ---
     apiVersion: "kubeadm.k8s.io/v1beta3"
     kind: ClusterConfiguration
     etcd:
@@ -104,7 +124,7 @@ this example.
             peerCertSANs:
             - "${HOST}"
             extraArgs:
-                initial-cluster: ${NAMES[0]}=https://${ETCDHOSTS[0]}:2380,${NAMES[1]}=https://${ETCDHOSTS[1]}:2380,${NAMES[2]}=https://${ETCDHOSTS[2]}:2380
+                initial-cluster: ${NAMES[0]}=https://${HOSTS[0]}:2380,${NAMES[1]}=https://${HOSTS[1]}:2380,${NAMES[2]}=https://${HOSTS[2]}:2380
                 initial-cluster-state: new
                 name: ${NAME}
                 listen-peer-urls: https://${HOST}:2380