Feature: allow url to be vagrant box type ubuntu/xenial64

[bjne]

Downloading boxes from a script is a bit of a hazzle now:

xenial=$(wget https://atlas.hashicorp.com/ubuntu/boxes/xenial64.json -O-|json_pp|grep '"url"'|head -1|cut -f4 -d")
vagrant mutate $xenial libvirt

Would be nice if you could support vagrant box type urls