Fix helm-toolkit snippet for statefulset updates

This fixes the incorrect statefulset update helm-toolkit
snippet, as the previous implementation used incorrect keys
and fields for statefulset updates. See below:

https://kubernetes.io/docs/tutorials/stateful-application/basic-stateful-set/#updating-statefulsets
https://github.com/kubernetes/kubernetes/issues/68397

Change-Id: I5044460f19f0c824b08f99a90472c2e4a0e53e52

helm-toolkit/templates/snippets/_kubernetes_upgrades_statefulset.tpl

@@ -1,9 +1,12 @@
 {{/*
 Copyright 2019 The Openstack-Helm Authors.
+
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at
+
    http://www.apache.org/licenses/LICENSE-2.0
+
 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
@@ -11,15 +14,40 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */}}
 
+{{/*
+abstract: |
+  Renders upgradeStrategy configuration for Kubernetes statefulsets.
+  See: https://kubernetes.io/docs/tutorials/stateful-application/basic-stateful-set/#updating-statefulsets
+  Types:
+    - RollingUpdate (default)
+    - OnDelete
+  Partitions:
+    - Stage updates to a statefulset by keeping pods at current version while
+      allowing mutations to statefulset's .spec.template
+values: |
+  pod:
+    lifecycle:
+      upgrades:
+        statefulsets:
+          pod_replacement_strategy: RollingUpdate
+          partition: 2
+usage: |
+  {{ tuple $envAll | include "helm-toolkit.snippets.kubernetes_upgrades_statefulset" | indent 2 }}
+return: |
+  updateStrategy:
+    type: RollingUpdate
+    rollingUpdate:
+      partition: 2
+*/}}
+
 {{- define "helm-toolkit.snippets.kubernetes_upgrades_statefulset" -}}
 {{- $envAll := index . 0 -}}
 {{- with $envAll.Values.pod.lifecycle.upgrades.statefulsets -}}
-revisionHistoryLimit: {{ .revision_history }}
-strategy:
+updateStrategy:
   type: {{ .pod_replacement_strategy }}
-  {{- if eq .pod_replacement_strategy "RollingUpdate" }}
+  {{ if .partition -}}
   rollingUpdate:
-    maxUnavailable: {{ .rolling_update.max_unavailable }}
-  {{- end }}
+    partition: {{ .partition }}
+  {{- end -}}
 {{- end -}}
 {{- end -}}