specify gerrit-committer

this is needed after https://review.openstack.org/#/c/28286 lands
to ensure the same upstream behavior.

Change-Id: I4f9110f7ecffa6652184c4f1f6ea5376f5b7c4a6
Reviewed-on: https://review.openstack.org/28287
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins