Make sudo user field required w/ a default

[mpdehaan]

If the sudo user is left blank, confusing error messages can result, that would be more clear if we didn't allow it to be blank.

I am proposing the sudo_user field on the worker pool default to "vespene_build" and be set to be non-nullable.

This value will not be used if using basic_container isolation, but will still be used for sudo isolation, making it more clear to new users that they need to change the username when trying things out for the first time.