Create GCE image from persistent disk instead of from a tarball.

The new flow:
1) Provision the instance
2) Tear down the instance, but keep the boot disk
3) Create an image from the disk
4) Tear down the disk

The step to update gcloud is no longer needed, since gceimagebundle isn't used anymore.
Fixes #1507 and addresses https://github.com/mitchellh/packer/issues/1447#issuecomment-61610235.