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This commit changes the behavior of `vagrant box outdated --global` so
it gets the latest version for the current machine's provider, rather
than the latest version for any provider.
