* make grpc: update Azure user task
* add retry_after to Azure user task
* add failure_count to Azure user task
* Backoff Azure VM installation attempts
Azure VMs that attempt to install will now be skipped in subsequent
discovery scans for a backoff period.
The backoff period is exponentially increasing up to a maximum.
* Fix flakey discovery test
The discovery config notification could be triggered by the config
watcher while the Azure watcher was running, which could sometimes
trigger the Azure watcher to run another fetch with the same config.
With this change, the config notification channel is drained when a
fetch is triggered.
Gavin Frazar
fe17a23ac4