Base parallel scale down implementation

[x13n]

Which component this PR applies to?

cluster-autoscaler

What type of PR is this?

/kind feature

What this PR does / why we need it:

Core part of https://github.com/kubernetes/autoscaler/issues/5079 implementation.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

This is a follow-up to https://github.com/kubernetes/autoscaler/pull/5229. Only last commit should be reviewed as a part of this PR. It will be on hold until the previous one is merged.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

/hold /assign @MaciekPytel @yaroslava-serdiuk

[x13n]

/hold cancel

[towca]

Thanks for addressing the comments, LGTM! Just 1 nit, feel free to unhold if you prefer not to address.

/lgtm /approve /hold

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: towca, x13n

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• ~~cluster-autoscaler/OWNERS~~ [towca]

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[x13n]

/hold cancel

[towca]

/lgtm