Use directly the box url instead of the metadata when downloading the vagrant box for flatcar linux

[ccojocar]

What type of PR is this?

/kind bug

What this PR does / why we need it:

It seems that vagrant resolve the relative path from the vagrant metadata to a local file path instead of URL.

See the URL in the metadata file https://lts.release.flatcar-linux.net/amd64-usr/current/flatcar_production_vagrant.json
This gets resolved to a file:// instead to a url:// due to https://github.com/hashicorp/vagrant/blob/87689f5f196cf31dea057eb91aa34a09949ebc8a/lib/vagrant/action/builtin/box_add.rb#L412.
It seems that there is not way to configure this currently.

Which issue(s) this PR fixes:

Does this PR have test?

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fix integration tests for Flatcar Linux.

[k8s-ci-robot]

Hi @ccojocar. Thanks for your PR.

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[JAORMX]

/lgtm cancel

let's wait for the flatcar job to pass first.

[ccojocar]

@saschagrunert This should fix the flatcar integration test.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ccojocar, JAORMX, saschagrunert

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