provider/google: include option to use source_disk with google_compute_image

[cblecker]

The google_compute_image resource currently only permits use of the raw_disk option, instead of either raw_disk, or source_disk per the API: https://godoc.org/google.golang.org/api/compute/v1#Image

The source_disk option would work better for Terraform configurations as it's something that can be passed from the self_link parameter of the google_compute_disk resource (raw_disk cannot be).

[ghost]

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