Non-empty buckets cause deletion to fail

[Henry Bell]

Currently a Terraform delete operation will fail once builds have been run because of non-empty GCS buckets (observed with the "artifacts" bucket). The force_destroy option, if applied to a bucket, will cause the bucket to be deleted along with the objects it contains, which will allow deletion of the resource:

https://www.terraform.io/docs/providers/google/r/storage_bucket.html#force_destroy

If this is considered unsafe then this functionality could be exposed with a top-level option (e.g. force_destroy_buckets or similar).

[Morgante Pell]

I think we should add this as a variable.