Add alert templates via alertmanager's values.yaml file

This adds the ability to define custom alert template via the
values.yaml file for Alertmanager. This will provide the ability
for an operator to define actions to be taken upon an alert firing
such as sending Slack alerts, email alerts, or any other
organization-specific action

Change-Id: I78a40e43cfeb7391699908a1f73b57846fedbcbb

prometheus-alertmanager/templates/configmap-etc.yaml

@@ -24,4 +24,6 @@ metadata:
 data:
   alertmanager.yml:
 {{- toYaml .Values.conf.alertmanager | indent 4 }}
+  alert-templates.tmpl:
+{{- toYaml .Values.conf.alert_templates | indent 4 }}
 {{- end }}

prometheus-alertmanager/templates/statefulset.yaml

@@ -72,6 +72,10 @@ spec:
           volumeMounts:
             - name: etc-alertmanager
               mountPath: /etc/config
+            - name: alertmanager-etc
+              mountPath: /etc/alertmanager/template/alert-templates.tmpl
+              subPath: alert-templates.tmpl
+              readOnly: true
             - name: alertmanager-etc
               mountPath: /etc/config/alertmanager.yml
               subPath: alertmanager.yml

prometheus-alertmanager/values.yaml

@@ -260,3 +260,4 @@ conf:
         room_id: 85
         message_format: html
         notify: true
+  alertmanager_templates: null