Update examples to include example of setting service account

[coryodaniel]

I was setting up a cluster with this module using the examples as a reference and kept running into issues about the service account not existing (I set up the project with the project factory).

As noted in the README¹ the node pools should specify the service account. I got tripped up on this since it wasn't in the examples.

It could be helpful to add a note in the examples or in the examples' README.

Looks like issue #23 would also resolve this.

Footnotes:

1. pardon the blame view, needed to link to the line in the readme

[morgante]

I think we should address this via #2 and simply always require that a Service Account be either created or passed in.

Relying on the default service account is brittle and unstable.

[aaron-lane]

Fixed by #80.