Migrate ubuntu-vuln-db.json.bz2 off of people.canonical.com

[mssalvatore]

The ubuntu vulnerability database JSON is currently hosted at https://people.canonical.com/~ubuntu-security/cvescan. This should be moved to https://security-metadata.canonical.com/~ubuntu-security. people.canonical.com will need to continue to host updated JSON for as long as v2.x.x versions are expected to continue to function.

[jdstrand]

Since we'd like to migrate off of people, I think that we'll want to consider redirects from people.c.c to security-metadata.c.c for anything that is expected to live on people.c.c.