Fix and update basic setup docs
Fix and update basic setup docs

* quickstart example config was missing required `image_project_id`
  to work at all (see #234, #228),
* update the GCP setup instructions to their current web UI as of now,
* update images versions to the current and available ones,
* reorganize initial setup sections for readability,
* change headings a bit for consistency,
* fix minor typos, replace http links with https etc.

mitchellh/vagrant-google