Fix broken build due to bad merge: restore waitCacheImages function.

[11janci]

Broken by #3737 (commit f9bf001).
The waitCacheImages() function called in the above commit was removed within commit 9edda72. This PR restores the function

[k8s-ci-robot]

Hi @11janci. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 11janci
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We suggest the following additional approver: dlorenc

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[11janci]

/assign @dlorenc

[minikube-bot]

Can one of the admins verify this patch?

[tstromberg]

@minikube-bot OK to test

[afbjorklund]

@11janci : thanks for fixing