[WIP] Reduce ec2:DescribeInstances call volume

[cartermckinnon]

What type of PR is this?

/kind feature

What this PR does / why we need it:

We call ec2:DescribeInstances way too often. This PR aims to reduce the call volume.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

The `ec2:DescribeInstances` API is called less often, with no change in behavior.

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[k8s-ci-robot]

This issue is currently awaiting triage.

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[k8s-ci-robot]

@cartermckinnon: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cloud-provider-aws-test	875f474	link	true	/test pull-cloud-provider-aws-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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