[stable/k8s-spot-termination-handler] Merge [incubator/kube-spot-termination-notice-handler] into this chart

[legal90]

What this PR does / why we need it:

Apparently, there are 2 charts for the same products in stable and incubator:

• incubator/kube-spot-termination-notice-handler added by [incubator/kube-spot-termination-notice-handler] New chart #1908
• stable/k8s-spot-termination-handler added by [stable/k8s-spot-termination-handler] AWS Spot termination handler #8187

This PR merges useful changes from [incubator/kube-spot-termination-notice-handler] to [stable/k8s-spot-termination-handler]:

• Switches to the image kubeaws/kube-spot-termination-notice-handler which has better support and versioning.
• Adds the support of Slack integration
• Allows to customize EC2 metadata poll interval
• Improves the RBAC policies, so there should be no issues for the k8s-spot-termination-handler to drain the node (the fix Kube-spot has to be able to run kubectl drain #4730 ported)
• Updates k8s labels according to the latest guidelines of Helm (https://github.com/helm/helm/blob/master/docs/chart_best_practices/labels.md#standard-labels)
• Deletes incubator/kube-spot-termination-notice-handler chart. Considered it as promoted to stable by this PR.

Checklist

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md

cc: @egeland , @KierranM

Closes #10270 as superseded

[legal90]

/assign @mattfarina

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Any further update will cause the issue/pull request to no longer be considered stale. Thank you for your contributions.

[legal90]

Could anyone please review this PR?
cc @foxish, @lachie83, @mattfarina

[egeland]

While I'm sad that my chart was gazumped ( 😉 ), this stable chart looks good, too! 👍

[egeland]

Can we add in #10093 functionality, too?

[egeland]

Would be good to get this done, so we have one canonical chart going forward.

Given this PR has lingered since 29Dec, I don't feel (too) guilty to mass-mention the approvers:
@lachie83
@prydonius
@sameersbn
@viglesiasce
@foxish
@unguiculus
@scottrigby
@mattfarina
@davidkarlsen
@paulczar
@cpanato

[legal90]

@egeland Thanks! I've appended changes from your PR #10093 to this one. 👍

[egeland]

Hopefully one of
@lachie83
@prydonius
@sameersbn
@viglesiasce
@foxish
@unguiculus
@scottrigby
@mattfarina
@davidkarlsen
@paulczar
@cpanato
will find a few minutes to approve this PR soon... ;)

[viglesiasce]

/ok-to-test
/lgtm

[k8s-ci-robot]

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