Provide node labels for webhook template

[imuqtadir]

Issue #, if available: #265

Description of changes: The node labels would be provided for users to use however by default we will not be sending out node labels. The users can now provide their webhook template and display the all/desired labels.

Sample:

// Display all node labels in raw format
Webhook Template: `{"text":"[NTH][Instance Interruption] EventID: {{ .EventID }} - Kind: {{ .Kind }} - NodeLabels: {{ .NodeLabels }} - Description: {{ .Description }} - Start Time: {{ .StartTime }}"}`

Webhook Content:
{"Content":"[NTH][Instance Interruption] EventID:  XXXX  - Kind: SCHEDULED_EVENT - NodeLabels: map[stage:prod beta.kubernetes.io/arch:amd64 beta.kubernetes.io/os:linux kubernetes.io/arch:amd64 kubernetes.io/hostname:ip-XXX-XXX-0-1.ec2.internal kubernetes.io/os:linux foo:bar] - Description: XXXX - Start Time: 2021-03-04 22:50:34 +0000 UTC"}

// Display all node labels in formatted form (change per your preference)
Webhook Template: `{"text":"[NTH][Instance Interruption] EventID: {{ .EventID }} - Kind: {{ .Kind }} - {{ range $key, $value := .NodeLabels }}{{ $key }}:{{ $value }} {{ end }} - Description: {{ .Description }} - Start Time: {{ .StartTime }}"}`

Webhook Content:
{"Content":"[NTH][Instance Interruption]  EventID: XXXX - Kind: SCHEDULED_EVENT - stage:prod beta.kubernetes.io/arch:amd64 beta.kubernetes.io/os:linux kubernetes.io/arch:amd64 kubernetes.io/hostname:ip-XXX-XXX-0-1.ec2.internal kubernetes.io/os:linux foo:bar - Description: XXXX   - Start Time: 2021-03-04 22:50:34 +0000 UTC"}

// Display specific label eg. stage label here
Webhook Template: `{"text":"[NTH][Instance Interruption] EventID: {{ .EventID }} - Kind: {{ .Kind }} - Stage: {{ .NodeLabels.stage }} - Description: {{ .Description }} - Start Time: {{ .StartTime }}"}`

Webhook Content:
{"Content":"[NTH][Instance Interruption] EventID: XXXX - Kind: SCHEDULED_EVENT - Stage: prod - Description: XXXX  - Start Time: 2021-03-04 22:50:34 +0000 UTC"}

// If the value of label is missing, we get <no value> but wouldn't fail webhook notification
Webhook Content:
{"Content":"[NTH][Instance Interruption] EventID: XXXX - Kind: SCHEDULED_EVENT - Stage: <no value> - Description: XXXX - Start Time: 2021-03-04 22:50:34 +0000 UTC"}

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

[bwagner5]

I think it will be good once NodeLabels is moved to interruptionEvent struct.

[bwagner5]

I made one little comment. But other than that it looks good!

[bwagner5]

LGTM! nicely done