(QENG-760) support for additional disk for vagrant

- if a given vagrant box has a disk_path then either create a new vmdk
  disk or use the provided disk
- defaults to 50 x 1024 bytes if the disk needs to created
According to the documentation, `size` is in MB. I recommend changing 500 to 5.

```
VBoxManage createhd         --filename <filename>
                            --size <megabytes>
                            [--format VDI|VMDK|VHD] (default: VDI)
                            [--variant Standard,Fixed,Split2G,Stream,ESX]
```