nebraska: Redirecting update URL

Update engine will be redirected to update using the locally running
nebraska when configs are updated.

```
// Start/stop verification.
localhost ~ # start nebraska
nebraska start/running, process 9092
localhost ~ # cat /mnt/stateful_partition/etc/lsb-release
CHROMEOS_AUSERVER=http://localhost:36343/update
localhost ~ # stop nebraska
nebraska stop/waiting
localhost ~ # cat /mnt/stateful_partition/etc/lsb-release
localhost ~ #

// Request redirection verification.
localhost ~ # update_engine_client --check_for_update
2021-08-12T00:01:57.708356Z INFO update_engine_client:
[update_engine_client.cc(513)] Initiating update check.
...
// Check update_engine.log
2021-08-12T00:01:57.714194Z INFO update_engine:
[omaha_request_action.cc(268)] Posting an Omaha request to
http://localhost:36855/update
```

BUG=b:184980277
TEST=# comment above

Change-Id: Ib7c36fea3205170ab8a833d6bf5088d9a0b5df91
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3089772
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: David Haddock <dhaddock@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
1 file changed