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.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mtrqq
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[k8s-ci-robot]

Hi @mtrqq. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mtrqq]

/assign towca