don’t check pre-release versions against known capz images

[jackfrancis]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR updates our logic which checks the Kubernetes version of a cluster E2E test spec against our known set of published test capz images: if the Kubernetes version has a pre-release on it, then we know we don't have an image for it and will rely upon downstream E2E foo to properly handle that as a "build of upstream k/k bits from an non-released commit".

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2052

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[jackfrancis]

/assign @CecileRobertMichon @mboersma

[CecileRobertMichon]

/lgtm
/approve
/cherry-pick release-1.1

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon

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[CecileRobertMichon]

/cherry-pick release-1.1

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: once the present PR merges, I will cherry-pick it on top of release-1.1 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.1

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[jackfrancis]

/retest

[jackfrancis]

/retest

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: new pull request created: #2103

In response to this:

/cherry-pick release-1.1

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.