We should actually just make these packer user variables as in @fnichol's original templates:

```
"iso_checksum": "{{user `iso_checksum`}}",
"iso_checksum_type": "md5",
"iso_url": "{{user `iso_url`}}",
```

Build command would like this then:
````
packer build -var iso_url=./path/to/local/iso iso_checksum=SOMEMD5 macosx-10.7.json
```