:bug: improve upper constraints handling

[Rozzii]

This commit:

• Provides a default value for the UPPER_CONSTRAINT docker file argument
• Changes the prepare-image.sh logic to check the actual size of the upper constraint file and not the presence of it.

Background of this change is that it has been noticed that on release branches during image building the "pinned" upper constraint file is not picked up rather pulled from upstream OpenStack because of a lack of default argument value during image building.

[Rozzii]

/retest

[Rozzii]

/test metal3-centos-e2e-integration-test-release-1-8 /test metal3-ubuntu-e2e-integration-test-release-1-8

[Rozzii]

/cc @tuminoid @elfosardo

[elfosardo]

/approve

[metal3-io-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: elfosardo

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/metal3-io/ironic-image/blob/release-26.0/OWNERS)~~ [elfosardo] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[tuminoid]

For reference: this is manual cherry-pick of https://github.com/metal3-io/ironic-image/pull/584

[tuminoid]

/retitle 🐛 improve upper constraints handling

[Rozzii]

/test metal3-centos-e2e-integration-test-release-1-8 /test metal3-ubuntu-e2e-integration-test-release-1-8

[tuminoid]

/retest

[Rozzii]

/ok-to-test