Introduce support for handling box architecture. Adds a new
`box_architecture` setting that defaults to `:auto` which will perform
automatic detection of the host system, but can be overridden with a
custom value. Can also be set to `nil` which will result in it fetching
the box flagged with the default architecture within the metadata.
Box collection has been modified to allow existing boxes already
downloaded and unpacked to still function as expected when architecture
information is not available.
This PR is dependent on the 2.1.0 release of the vagrant_cloud
library. It updates the `cloud` command to use the new interface
for interacting with the Vagrant Cloud API. It also adds support
for direct to backend storage uploads, and defaults to this
method.
Also included is a bit of cleanup refactoring, addition of method
documentation, and fixing up some small issues around custom username
usage within the internal client for authentication.
