* Add subnetwork_project field to allow for XPN in GCE instance templates
* Missing os import
* Removing unneeded check
* fix formatting
* Add subnetwork_project to read
Update our instance template to include metadata_startup_script, to
match our instance resource. Also, we've resolved the diff errors around
metadata.startup-script, and people want to use that to create startup
scripts that don't force a restart when they're changed, so let's stop
disallowing it.
Also, we had a bunch of calls to `schema.ResourceData.Set` that ignored
the errors, so I added error handling for those calls. It's mostly
bundled with this code because I couldn't be sure whether it was the
root of bugs or not, so I took care of it while addressing the startup
script issue.
