only and except support for provisioners

[mohae]

Add support for only and except:

{
  "type": "shell",
  "script": "script.sh",
  "only": ["virtualbox-iso"]
}

From https://www.packer.io/docs/templates/provisioners.html
The values for `only` and `except` are _build names_, not builder types.

[mohae]

Only and except support exists. The user needs to ensure the correct values are used.