Add "include_data_disks" option to control whether data disks are included in the image

[Jalle19]

Fixes #141

Enables e.g. the following use case:

// Use a data disk during baking but don't include it in the final image
include_data_disks = false
data_disks {
  disk_type = "CLOUD_BSSD"
  disk_size = 20
}

I could not find a way to correlate the data disks as returned by the API with our template data disk definitions, so it's difficult if not impossible to selectively exclude data disks from the image. While a global option is not the most flexible, it at least solves the problem where you have a single data disk that you don't want to include in the image.

This PR includes parts of #133 and #135 so it can be applied cleanly some day.

[Jalle19]

Switched to using a Trilean so we can properly default to true