updater: Set vulns' Severity from NVD metadata fetcher if unknown
updater: Set vulns' Severity from NVD metadata fetcher if unknown

If a Vulnerability that goes through the NVD metadata fetcher has an
empty or Unknown Severity, then use the CVSS score to set one. This
will help to get a more consistent database when a vulnerability source
does not provide this information.

jgsqware/clair