Improve project variable definitions

As part of the work on gcp-cloud-run-app#47 (closed) I found a couple of small issues with the way that this module allows projects to be provided. This MR aims to make it clearer when specifying separate projects for the alert policies - such as our "meta projects" being configured as Cloud Monitoring "scoping" projects - and the other auth proxy resources.

We will also continue to support specifying the project via the default google provider object. However, I have removed the data "google_project" calls and replaced them with a single data.google_client_config call as this is the correct way to access the default provider information.