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.

When expanding the box url, prefer the API endpoint which is updated to
include provider architecture information. Test the API endpoint and the
legacy endpoint and use which ever is valid, with the API taking
precedence. This allows Vagrant to continue with non Vagrant Cloud
servers that do not implement the API endpoint.

With the initial layout of `provider/architecture`, after installing a
box with architecture support downgrading Vagrant would result in it
being unable to process the box collection. Swapping the layout to be
`architecture/provider` allows downgrades to still properly process the
box collection.

When the reported architecture is unknown and the provider is listed as
the default architecture, add the box without architecture information
so it is installed without architecture information on the path within
the collection.