Consider pods with emptydir volume in memory be evictable

[askoriy]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Currently, pods with in-memory storage are considered as having local storage and are not evictable. This prevents cluster autoscaling to down-scale the node if such pods are present.

In-memory storage is widely used for organizing init or sidecar container communication and in the vast majority of cases should not contain critical data to be lost (see details explanation at #3984).
If the new default behavior is not desired then annotation could be added: "cluster-autoscaler.kubernetes.io/safe-to-evict": "false"

Which issue(s) this PR fixes:

Fixes #3984, #2048

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Pods having `emptyDir` volumes with `medium: Memory` are not considered to have local storage and therefore are evictable

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: askoriy / name: Oleksandr Skoryi (946189f)

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[towca]

This isn't a backwards compatible change (CA would start removing existing pods/nodes that were previously unremovable), so I wouldn't backport it anywhere. It should be fine for a new minor release, and it definitely makes sense. Thanks for the contribution!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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