builder/googlecompute: configure instance name

Retain the current default of an instance name generated by UUID, but allow
users to choose a specific one. Useful for provisioning with Puppet when a
node name is used to select the right manifest.

This is possibly something useful on all builders, but I started here to see if it was a change you were interested in first.

Let me know if you think this needs additional tests. I also wasn't sure if you wanted to PR to include an entry for CHANGELOG or if you prefer to handle that - happy to adjust.