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.
Some google resources required network be refernced by resource URL (aka self_link), while others required network name.
This change allows either to be supplied.
DRY it out, and add a fix for #5552.
Auto-generating an Instance Template name (or just its suffix) allows the
create_before_destroy lifecycle option to function correctly on the
Instance Template resource. This in turn allows Instance Group Managers
to be updated without being destroyed.