Additional Disk Support for vmware-iso Builder

I'd like to reopen the discussion from #703 about adding support for specifying additional disks during Packer's build phase.  Unlike the Virtualbox builder, I don't think there's a vboxmanage-type workaround here.

My use case is that I'm packaging VM appliances, and need to end up with a system disk mounted at `/` and a disk for user data mounted at `/data`. In a previous iteration we used VMware Studio, and multiple disk support was one of the few things it did well.  

I went ahead and implemented the same syntax as was used in the earlier issue, but I'm definitely open to a discussion on what additional things you might want to specify about the disks.  For instance, I could definitely see an object that specifies both name and size:

```
    "additional_disks": [
        {
            "name": "data_disk",
            "size": "204800"
        }
    ]
```

I'm going to guess that this use case does come up for other people, so if we could unobtrusively come up with enough of an implementation to handle the basic case and get out of the way for those who don't need it, that would be awesome.