lava_wait_jobs: fetch_artifacts: Use exponential backoff

And start with 3s delay. All to actually give LAVA chance to recover between
retries, because otherwise in https://linaro.atlassian.net/browse/STG-4617
we saw case when retries don't really help much.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Idf8934f10dff62d7734ccdb7909c37102c38c816
diff --git a/lava_helper/lava_wait_jobs.py b/lava_helper/lava_wait_jobs.py
index 305f4b7..4a14fe0 100755
--- a/lava_helper/lava_wait_jobs.py
+++ b/lava_helper/lava_wait_jobs.py
@@ -96,6 +96,7 @@
         t = time.time()
         _log.info("Fetching artifacts for job %d to %s", job_id, job_dir)
 
+        retry_delay = 3
         for retry in range(3, 0, -1):
             try:
                 os.makedirs(job_dir, exist_ok=True)
@@ -117,7 +118,8 @@
                     raise
                 else:
                     _log.warning("fetch_artifacts: Error %r occurred, retrying", e)
-                    time.sleep(2)
+                    time.sleep(retry_delay)
+                    retry_delay *= 2
 
         _log.info("Fetched artifacts in %ds", time.time() - t)
         codecov_helper.extract_trace_data(target_log, job_dir)