CBQE-962 Clean up previous data in seriesly db  when celery starts

When celery starts, we need to clean up previous data stored
in seriesly database from last run.

Change-Id: I80e0daf23f56d6cdc49d70abb56a95020fde7d84
Reviewed-on: http://review.couchbase.org/23761
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>