Safer default VM names for Virtualbox

If the same Vagrantfile is up'd in the same second, in the same basedir, there is a conflict in the name provided to Virtualbox.

It may sound rare, but all Jenkins jobs sit in a directory named  `workspace/` and if they're listening for the same trigger, they'll run into this issue.

This changes the name suffix from seconds to milliseconds and + `rand(100000)`