Add Vultr disk artifacts to stream metadata

[dghubble]

• FCOS Jenkins pipeline builds Vultr disk images, but they
  need to be mentioned in stream metadata too
• Support for Vultr platform fedora-coreos-tracker#355

[dghubble]

I believe the expectation is that this generator feeds into https://github.com/coreos/fedora-coreos-streams, which provides some clues on how this should be validated.

go build .
./fedora-coreos-stream-generator -releases=https://builds.coreos.fedoraproject.org/prod/streams/testing/releases.json -pretty-print

This shows that releases.json doesn't contain vultr artifacts, but the Jenkins pipeline produces them and lists them in metadata (e.g. https://builds.coreos.fedoraproject.org/prod/streams/testing/builds/32.20200601.2.1/x86_64/meta.json).

So something else is needed to update releases.json I think.

[dghubble]

I think its coreos/fedora-coreos-releng-automation#108 which would be needed