Adds adds a wait operation for deployments to measurement-lab

A production build failed with this error:

> Cannot operate on apps/measurement-lab because an operation is already in progress for apps/measurement-lab

Looking at the build step timing compared to the operations in App Engine,
there is a very tight timing issue. This commit adds a new build step that
waits for the most recent pending operation to complete before moving on to
deploying dispatch.yaml. I _believe_ this should safeguard against this
particular error condition in the build.

cloudbuild.yaml

@@ -33,4 +33,6 @@ steps:
   env:
   - PROJECT_IN=measurement-lab
   args:
+  - gcloud app operations wait $(gcloud app operations list --format="value(id)" --pending --limit=1) || true
   - gcloud app deploy grafana-public/dispatch.yaml --project $PROJECT_ID
+