Add a comment to keep etcd name in sync and change hostname to HOSTNAME

Author: wenjiaswe

cluster/gce/manifests/etcd.manifest

@@ -53,6 +53,9 @@
       },
       { "name": "ETCD_SNAPSHOT_COUNT",
         "value": "10000"
+      },
+      { "name": "ETCD_HOSTNAME",
+        "value": "{{ hostname }}"
       }
         ],
     "livenessProbe": {

cluster/images/etcd/migrate-if-needed.sh

@@ -46,7 +46,8 @@ set -o nounset
 BUNDLED_VERSIONS="3.0.17, 3.1.12, 3.2.24, 3.3.17, 3.4.7"
 
 # shellcheck disable=SC2039
-ETCD_NAME="${ETCD_NAME:-etcd-${ETCD_HOSTNAME:-$(hostname -s)}}"
+# NOTE: Make sure the resulted ETCD_NAME agrees with --name in etcd.manifest: https://github.com/kubernetes/kubernetes/blob/e7ca64fbe16d0c4b6c7b36aecde9cd75042b2828/cluster/gce/manifests/etcd.manifest#L27
+ETCD_NAME="${ETCD_NAME:-etcd-${ETCD_HOSTNAME:-$HOSTNAME}}"
 if [ -z "${DATA_DIRECTORY:-}" ]; then
   echo "DATA_DIRECTORY variable unset - unexpected failure"
   exit 1