Vagrant Cloud provides support for uploading directly to the backend
storage instead of streaming through Vagrant Cloud. This adds support
for direct to storage uploads and sets it as the default upload method.
A new option has been added to disable this behavior and revert back
to streaming upload via Vagrant Cloud (`no_direct_upload`).
This default for uploading directly to the backend storage also matches
up with changes being added to Vagrant proper for box upload behavior:
hashicorp/vagrant#11916
Megan Marsh
Chris Roberts