Prior to this commit, providers like docker would fail to be brought up
because they do not store box objects like virtualbox or vmware
provider guests. This commit fixes that by making sure the box object
exists before writing the metadata file to disk.
When a guest is created, the box metadata information is stored in the
machine data directory. This allows modifications to happen to the
Vagrantfile definition of the box in use (box name change, box version
change, etc) while still allowing the Machine instance of an active
guest successfully load the box currently backing it.