Default to alpha dynamic PV annotation

Addresses #227
Alpha dynamic provisioner only looks for annotation, not value. To
enable the beta provisioner, operator provides
`.Values.{alertmanager|server}.persistentVolume.storageClass`.

stable/prometheus/templates/alertmanager-pvc.yaml

@@ -3,6 +3,11 @@ apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
   annotations:
+  {{- if .Values.alertmanager.persistentVolume.storageClass -}}
+    volume.beta.kubernetes.io/storage-class: {{ .Values.alertmanager.persistentVolume.storageClass }}
+  {{ else }}
+    volume.alpha.kubernetes.io/storage-class: ''
+  {{- end }}
   {{- range $key, $value := .Values.alertmanager.persistentVolume.annotations }}
     {{ $key }}: {{ $value }}
   {{- end }}

stable/prometheus/templates/server-pvc.yaml

@@ -3,6 +3,11 @@ apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
   annotations:
+  {{- if .Values.server.persistentVolume.storageClass -}}
+    volume.beta.kubernetes.io/storage-class: {{ .Values.server.persistentVolume.storageClass }}
+  {{ else }}
+    volume.alpha.kubernetes.io/storage-class: ''
+  {{- end }}
   {{- range $key, $value := .Values.server.persistentVolume.annotations }}
     {{ $key }}: {{ $value }}
   {{- end }}

stable/prometheus/values.yaml

@@ -56,13 +56,16 @@ alertmanager:
 
     ## AlertManager data Persistent Volume annotations
     ##
-    annotations:
-      volume.beta.kubernetes.io/storage-class: standard
+    # annotations:
 
     ## AlertManager data Persistent Volume size
     ##
     size: 2Gi
 
+    ## AlertManager data Persistent Volume Storage Class
+    ##
+    # storageClass:
+
   ## Alertmanager resource requests and limits
   ## Ref: http://kubernetes.io/docs/user-guide/compute-resources/
   ##
@@ -168,13 +171,16 @@ server:
 
     ## Server data Persistent Volume annotations
     ##
-    annotations:
-      volume.beta.kubernetes.io/storage-class: standard
+    # annotations:
 
     ## Server data Persistent Volume size
     ##
     size: 8Gi
 
+    ## AlertManager data Persistent Volume Storage Class
+    ##
+    # storageClass:
+
   ## Server resource requests and limits
   ## Ref: http://kubernetes.io/docs/user-guide/compute-resources/
   ##