Vagrantfile: Reference the remote qcow2 box url

Depends on !29 (closed)

Test Plan

vagrant up qemutest (downloads the qcow2 url and builds the box fine)

vagrant up qemutest
Bringing machine 'qemutest' up with 'libvirt' provider...
==> qemutest: Box 'debian10-20201010-1039' could not be found. Attempting to find and install...
    qemutest: Box Provider: libvirt
    qemutest: Box Version: >= 0
==> qemutest: Box file was not detected as metadata. Adding it directly...
==> qemutest: Adding box 'debian10-20201010-1039' (v0) for provider: libvirt
    qemutest: Downloading: https://annex.softwareheritage.org/public/isos/libvirt/debian/swh-debian-10.6-amd64-20201010-1039.qcow2
    qemutest: Progress: 32% (Rate: 8763k/s, Estimated time remaining: 0:00:41)
...

To reproduce if you have already data in your users:

rm -rf ~/.vagrant.d $SWH_PUPPET_ENVIRONMENT_HOME/.vagrant

Note: a tad violent, maybe using a more delicate rm instruction would be best ;)

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Migrated from D4226 (view on Phabricator)