Add a parameter 'registry_project_id' to the GKE module templates

[mmontan]

The PR allows configuring the project holding the GCR registry when
used in connection with 'create_service_account'=true and
grant_registry_access=true.

Holding the GCR is a project with other resources increases the risk of
exposing sensitive data to the service account running the nodes, as the
required permissions of role roles/storage.objectViewer provide access
to all storage objects in the project.

[morgante]

The core change looks fine.

For testing, could we consider adding these tests to an existing suite? GKE tests are unfortunately very long so I'd like to avoid the proliferation of test suites so we don't make integration tests run even longer than they already do.

[mmontan]

The Cloud Build check pass now. I can't access logs from the concourse builds, so it's hard to debug them. How do I find the logs?

[morgante]

I can't access logs from the concourse builds, so it's hard to debug them.

You can ignore Concourse tests. They're legacy and no longer needed.