Fix memory leak while fetching GCE instance templates.

[mtrqq]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR prevents instance templates and instance template names cache from leaking. Currently those caches aren't cleaned at all which causes issues for clusters with MIGs churn. This change is are adding cleaning for those caches to RegenerateMigInstancesCache which is called each hour worst case and cleans up caches only for the MIGs which are untracked.

Does this PR introduce a user-facing change?

Fixed memory leak for GCE instance templates fetching

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

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[mtrqq]

/assign towca

[towca]

Ughh, these caches are such a pain to read through or extend :/

Thanks for the change! Have you tested that it fixes the leak in a real cluster?

[towca]

k8s.io/utils/set.Set[GceRef] will read much nicer here.

[mtrqq]

Agree, that would be nice, but GceRef doesn't comply with the requirements of set.Set, due to not being ordered.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mtrqq
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[mtrqq]

Ughh, these caches are such a pain to read through or extend :/

Thanks for the change! Have you tested that it fixes the leak in a real cluster?

I've tested initial change in the cluster and it was working as expected, while porting the code to Refresh method it's very likely that I'll need to retest it