Fix bug in retry-forever usage.

- Push retry-forever wrapping to curl invocations.
- Collect curl retry flags into a single variable.
- Remove 'sudo: false' in master.yaml, is unnecessary and breaks older
  cloud-init versions.
- Change log-error status reason to be more accurate.
- Fix the some 'python' invocations to 'python3'.

cluster/gce/util.sh

@@ -1911,7 +1911,7 @@ function update-or-verify-gcloud() {
   else
     local version
     version=$(gcloud version --format=json)
-    python -c"
+    python3 -c"
 import json,sys
 from distutils import version