Cherry pick #7578 into release 1.14

[mitch000001]

This cherry-picks #7578 , a bugfix for the OpenStack cloud provider.
It uses the specified subnets to roll out the instances. Previously it wasn't using them which could lead to cluster instances not coming up correctly.

[k8s-ci-robot]

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[zetaab]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mitch000001, zetaab

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[zetaab]

/ok-to-test