Add back IMAGE_OS_VERSION in Cloudstack buildspec

[abhay-krishna]

We need to set this so that correct ISO URL and ISO checksum are assigned for constructing the Cloudstack configuration file for image-builder.

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[abhay-krishna]

/cherrypick release-0.19

[eks-distro-pr-bot]

@abhay-krishna: once the present PR merges, I will cherry-pick it on top of release-0.19 in a new PR and assign it to you.

In response to this:

/cherrypick release-0.19

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[eks-distro-pr-bot]

@abhay-krishna: new pull request created: #2963

In response to this:

/cherrypick release-0.19

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