In the case that not provider is given then whatever provider the box
represents will be added to the system. Ideally, a provider will be
given, but if not, Vagrant still does a "best effort" to install the
box.
This involved defaulting all box searching at the moment to VirtualBox.
Additionally, box upgrading is not yet handled. This needs to be done at
some point.
This is the beginning of the new box internals. The basic idea is that
the new box has a new field: provider. The provider will describe what
provider that box was built with and what provider it is made to work
with.
