Add an option for buildextend-gcp

[mrguitar]

Currently setting the platform id for gce works well for one off image creation. e.g. gf-platformid rhcos-410-...-qemu.qcow2 rhcos-410-...-gce.qcow2 gce

We should add the complete image format needs for GCP as well.

Here's what I'm using to make this work:
(this was taken from Peter's work here: https://github.com/pschiffe/gce-system-container)

qemu-img convert -p -S 4096 -f qcow2 -O raw rhcos-gce.qcow2 disk.raw
tar -Szcvf rhcos.tar.gz disk.raw
BUCKET_NAME=coreos-test-image-bucket
gsutil mb gs://${BUCKET_NAME}
gsutil -o GSUtil:parallel_composite_upload_threshold=150M cp rhcos.tar.gz gs://${BUCKET_NAME}
gcloud compute images create rhcos-20190418 --source-uri gs://${BUCKET_NAME}/rhcos.tar.gz
gsutil -m rm -r gs://${BUCKET_NAME}
gcloud compute instances create "rhcos" --zone us-central1-a --machine-type n1-standard-1 --image "rhcos-20190418" --boot-disk-size "30" --boot-disk-type "pd-ssd" --metadata-from-file user-data=gcp.ign