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.
Within GCE, images may be shared across projects. Prior to this
commit, there was no way to inform the GCE builder that a source
image belonged to a specific project. This adds an optional
'source_image_project_id' key to the GCE builder config.
