Add a utility script for mass-comments.

The script prints a gerrit review command for every change in
a given zuul pipeline.

Change-Id: Ibdee87ea64c977e7589cbc3ee9567e866cb78341
Reviewed-on: https://review.openstack.org/23784
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins