cbuildbot: Make the PaygenStage a RetryStage.

The PaygenStage continues to have occasional flake around loopback device
mounting and cleanup. Retry the entire stage to work around this.

Also increase the timeout used to wait for PUSHIMAGE. I reverting the first
version of this CL thinking it was causing issues, when the real problem
was the timeout being too short.

BUG=chromium:369566
TEST=Unitests + sanity trybot

Change-Id: Id1e247facea10e355c2d742c074628fe4dd719e1
Reviewed-on: https://chromium-review.googlesource.com/200199
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 8aa76d0..67005ba 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -521,7 +521,8 @@
 
     stage_list += [
         [stages.SignerTestStage, board, archive_stage],
-        [stages.PaygenStage, board, archive_stage],
+        [generic_stages.RetryStage, 1,
+         stages.PaygenStage, board, archive_stage],
         [test_stages.UnitTestStage, board],
         [artifact_stages.UploadPrebuiltsStage, board],
         [artifact_stages.DevInstallerPrebuiltsStage, board],