Allow users to configure webhook message content with a template file

[supasteev0]

Description of changes:

When the destination of the webhook requires a more complex message content, it can be easier to provide a json template file which will be mounted into the pod, rather than having the webhook message content set in an inline variable.

This will allow users to set a WEBHOOK_TEMPLATE_FILE environment variable (--webhook-template-file argument) to set the template file path which will be used to send the webhook message. This takes precedence over the --webhook-template argument.

Template file will have to exist (be mounted into the pod as volume by users) otherwise it will throw an error.

Example:

---
apiVersion: v1
data:
  webhook-alert-template.json: |-
    {
      "status": "firing",
      "alerts": [
          {
            "status": "firing",
            "labels": {
              "alertname": "myalert",
              "region": "eu-west-1",
              "severity": "high"
            }
          }
       ]
    }
kind: ConfigMap
metadata:
  name: webhook-template
  namespace: monitoring
---
apiVersion: apps/v1
kind: DaemonSet
metadata:
  labels:
    app: aws-node-termination-handler
  name: aws-node-termination-handler
  namespace: monitoring
spec:
  selector:
    matchLabels:
      app: aws-node-termination-handler
  template:
    metadata:
      labels:
        app: aws-node-termination-handler
    spec:
      containers:
      - env:
        ...
        - name: WEBHOOK_TEMPLATE_FILE
          value: /config/webhook-alert-template.json
        ...
        volumeMounts:
        - mountPath: /config/
          name: template
      volumes:
      - configMap:
          name: webhook-template
        name: template

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #253 into master will decrease coverage by 1.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
- Coverage   81.60%   80.56%   -1.04%     
==========================================
  Files          10       10              
  Lines         799      818      +19     
==========================================
+ Hits          652      659       +7     
- Misses        131      141      +10     
- Partials       16       18       +2     

Impacted Files	Coverage Δ
pkg/webhook/webhook.go	77.46% <48.00%> (-14.99%)	⬇️
pkg/config/config.go	100.00% <100.00%> (ø)

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[bwagner5]

Rather than making the user mount the file, can you instead use a ConfigMap similar to the way the webhookURLSecretName works.

[supasteev0]

@bwagner5 I have update the Helm chart by adding 2 new variables in order to set the configmap name and the template file name that will be mounted into the pod.

[bwagner5]

This looks good! I missed one thing during my first pass review.

Can you update the ValidateWebhookConfig() function as well? The validate function is called on startup so you know if your template is invalid upfront rather than waiting for a termination event for the error to be thrown.

https://github.com/aws/aws-node-termination-handler/blob/master/pkg/webhook/webhook.go#L103

[supasteev0]

I updated the ValidateWebhookConfig function as requested. I also fixed a typo on the nthConfig variable which was written in 2 different ways. Looks like the travis test has failed because of an issue with ec2Mock not responding, but I don't know how to re trigger the job.

[bwagner5]

LGTM! 🚀 thanks for the contribution!