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
1 file changed