Fixes the following vet reports:
builder/googlecompute/step_create_instance_test.go:42: possible formatting directive in Fatal call
builder/googlecompute/step_teardown_instance_test.go:29: possible formatting directive in Fatal call
builder/googlecompute/step_teardown_instance_test.go:39: possible formatting directive in Fatal call
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.
Matthew Hooker
Emil Hessman
Dustin Carlino