Allow VirtualBox base folder to be overridden from a profile file.

This allows someone via the $HOME/.boot2docker/profile file to override where VirtualBox puts its VM directory in case you don't want it going to the default location that VirtualBox images are always placed for your VirtualBox configuration.

For example, can in the $HOME/.boot2docker/profile file now have:

```
VM_NAME=boot2docker-vm
VM_DIRECTORY=$HOME/.boot2docker
VM_DISK=$VM_DIRECTORY/boot2docker.vmdk
BOOT2DOCKER_ISO=$VM_DIRECTORY/boot2docker.iso
```

That way everything can be kept under the $HOME/.boot2docker directory.