This commit tests whether an interface is nil before type asserting it
to string - this should fix the panic reported in #8609.
We also clean up the schema definition to the newer style without
redundant type declarations.
Creating most google cloud resources uses the compute_operation to
wait for the creation to complete. However, the computeOperationWait*
functions always uses the global `config.Project`, instead of the resource-
specific one.
This means that creating resource in a project other than the main one
fails with a 404 on the operation resource.
This patch uses the project from google_compute_instance instead of the
global one.
This is the first step in removing the config dependency on "project".
This change is backwards-compatible because the value for this new
attribute defaults to the value from the provider.
