Simplify interface of Google examples and fix the name and comment of userName variable

A standard tool gcloud creates these files and puts them in these locations (see https://developers.google.com/compute/docs/instances#sshkeys). Situations when they are located elsewhere should be very rare. I think that it's very inconvenient for a user having to enter the paths each time as command-line arguments, this is why I suggest hardcoding the locations in this example.