Use direct references rather than interpolated strings for custom roles

[zefdelgadillo]

In order for Terraform to apply resources in the correct order, IAM configuration should use the direct attributes from the custom role definition.

See role below:

resource "google_project_iam_binding" "BigQuerySchemaUpdate" {
  project = var.tag_engine_project
  role    = "projects/${var.bigquery_project}/roles/BigQuerySchemaUpdate"**
  members = ["serviceAccount:${var.tag_creator_sa}"]
  depends_on = [google_project_service.tag_engine_project]
}