This seemed to be missing from the googlecompute provider. Now if the
ssh_private_key_file is provided, that will be used in place of a temporary
key. I didn't update the googlecompute specific docs under `./website/`, since
this parameter is already documented under the communicators templates page.
- Startup scripts can be provided through the instance creation metadata field 'startup-script'.
- Script log can be copied to a GCS location by setting the metadata field 'startup-script-log-dest'.
Added Retry method to googlecompute package.
Added GetSerialPortOutput to googlecompute Drivers.
Added StepWaitInstanceStartup (and associated test) which waits for an
instance startup-script to finish.
Changed the instance service account to use the same service account as the one provided in the Packer config template. It was the project default service account.
Tested googlecompute package with 'go test' and also performed builds
with a startup script and without a startup script.
Lars Wander
Scott Crunkleton
Chris Bednarski
Ross Smith II
Mitchell Hashimoto
Prasanna Santhanam
Kelsey Hightower