default to SCSI disks not virtio

This means less to edit manually for Windows users.  Eventually
we might also support other hypervisors which can't do virtio.
default to nova-multi-compute-qemu

This means less to edit manually for Windows users.  Eventually
we might also support other hypervisors which can't do nested
virtualization.
Horizon needs nova :-(
move shell code out of Vagrantfile
require acceptance of EULA on first ssh